LMP8358MT

LMP8358 Zero-Drift, Programmable Instrumentation Amplifier with Diagnostics May 5, 2010 LMP8358 Zero-Drift, Programmable Instrumentation Amplifier with Diagnostics General Description The LMP8358 is a precision programmable-gain instrumentation amplifier in National's LMP® precision amplifier family. Its gain can be programmed to 10, 20, 50, 100, 200, 500, or 1000 through an SPI-compatible serial interface or through a parallel interface. Alternatively, its gain can be set to an arbitrary value using two external resistors. The LMP8358 uses patented techniques to measure and continuously correct its input offset voltage, eliminating offset drift over time and temperature and the effect of 1/f noise. Its ground-sensing CMOS input features a high CMRR and low input bias currents. It is capable of sensing differential input voltages in a commonmode range that extends from 100mV below the negative supply to 1.4V below the positive supply, making it an ideal solution for interfacing with ground-referenced sensors, supply-referenced sensor bridges, and any other application requiring precision and long-term stability. Additionally, the LMP8358 includes fault detection circuitry to detect open and shorted inputs and deteriorating connections to the signal source. Other features that make the LMP8358 a versatile solution for many applications are its rail-to-rail output, low input voltage noise and high gain-bandwidth product. Features Typical Values unless otherwise noted, TA = 25°C 2.7V to 5.5V ■ Supply voltage 1.8 mA ■ Supply current 0.15% ■ Max gain error 16 ppm/°C ■ Max gain drift 110 dB ■ Min CMRR 10 µV ■ Max offset voltage 50 nV/°C ■ Max offset voltage drift 8 MHz ■ GBW (gain = 10) 100 ppm ■ Max non-linearity −40°C to 125°C ■ Operating temperature range ■ Input fault detection ■ SPI or pin configurable modes 92 dB ■ EMIRR at 1.8GHz ■ 14-Pin SOIC and 14-Pin TSSOP Package Applications ■ ■ ■ ■ ■ Bridge sensor amplifier Thermopile amplifier Portable instrumentation Medical instrumentation Precision low-side current sensing Typical Application 300454g1 LMP® is a registered trademark of National Semiconductor Corporation. © 2010 National Semiconductor Corporation 300454 www.national.com LMP8358 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model Machine Model Charge Device VIN Differential (V+IN − V−IN) Output Short Circuit Duration (Note 3) Any pin relative to V− +IN, −IN, OUT Pins +IN, −IN Pins 2kV 200V 1kV VS 6V, −0.3V V+ +0.3V, V− −0.3V ±10 mA Storage Temperature Range −65°C to 150°C Junction Temperature (Note 4) 150°C For soldering specifications: see product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf Operating Ratings Temperature Range Supply Voltage (VS = V+ – V−) VIN Differential (V+IN − V−IN) (Note 1) −40°C to 125°C 2.7V to 5.5V ±100mV 145°C/W 135°C/W Package Thermal Resistance (θJA (Note 4) 14-Pin SOIC 14-Pin TSSOP 3.3V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 3.3V , V− = 0V, VREF = V+/2, VCM = V+/2, RL = 10 kΩ to VREF, CL = 10 pF; Serial Control Register: G[2:0] = 110b (Gain = 1000x), COMP[2:0] = 000b, MUX[1:0] = 00b, POL, SHDN, FILT, PIN = 0b, CUR[2:0] = 000b. Boldface limits apply at the temperature extremes. Symbol VOS Parameter Input Offset Voltage VCM = V+/2 VCM = 0V TCVOS CMRR CMVR Input Offset Voltage Temperature Drift VCM = V+/2 (Note 8) VCM = 0V Common Mode Rejection Ratio Common Mode Voltage Range V− − 0.1V ≤ VCM ≤ V+ − 1.4V CMRR ≥ 110 dB V− + 0.1V ≤ VREFF ≤ V+ − 1.4V 110 105 −0.1 110 105 112 105 145 138 83 93 50||1 50||1 ±100 0.006 0.1 Gain = 10, f = 1 kHz Gain = 20, f = 1 kHz Gain = 50, f = 1 kHz Gain = 100, f = 1 kHz Gain = 200, f = 1 kHz Gain = 500, f = 1 kHz Gain = 1000, f = 1 kHz Gain = External, f = 1 kHz 27 31 28 27 28 28 27 27 nV/ 1.2 2 112 139 1.9 Conditions Min Typ Max (Note 6) (Note 5) (Note 6) 1 1 10 15 10 15 50 50 Units µV nV/°C dB V dB VREFRR VREF Rejection Ratio PSRR EMIRR ZINDM ZINCM VINDM IB IOS en Power supply Rejection Ratio Electro Magnetic Interference Rejection Ratio Differential Input Impedance Common Mode Input Impedance Differential Mode Input Voltage Input Bias Current Input Offset Current Input Voltage Noise Density 2.7V ≤ V+ ≤ 5.5V +IN / −IN, VRF = 100 mVP, f = 900 MHz +IN / −IN, VRF = 100 mVP, f = 1800 MHz dB dB MΩ ‖ pF MΩ ‖ pF mV nA pA www.national.com 2 LMP8358 Symbol en Parameter Input Voltage Noise Conditions Gain = 10, 0.1 Hz < f < 10 Hz Gain = 20, 0.1 Hz < f < 10 Hz Gain = 50, 0.1 Hz < f < 10 Hz Gain = 100, 0.1 Hz < f < 10 Hz Gain = 200, 0.1 Hz < f < 10 Hz Gain = 500, 0.1 Hz < f < 10 Hz Gain = 1000, 0.1 Hz < f < 10 Hz Gain = External, 0.1 Hz < f < 10 Hz Min Typ Max (Note 6) (Note 5) (Note 6) 0.9 0.6 0.6 0.7 0.6 0.6 0.6 0.6 0.5 0.03 0.03 0.03 3 16 0.1 0.15 0.15 0.25 Units µVPP In GE GE GE Input Current Noise Density Gain Error Gain Error Gain Error Contribution from Chip Gain Error Temperature Coefficient Gain = 100, f = 1 kHz Gain = 10, 20, 50, 100, 200, 500 VOUT = VREF + 1V and VOUT = VREF − 1V Gain = 1000 VOUT = VREF + 1V and VOUT = VREF − 1V VOUT = VREF + 1V and VOUT = VREF − 1V For all gain settings (internal and external), VOUT = VREF + 1V and VOUT = VREF − 1V COMP[2:0] =000b, Gain > 10 COMP[2:0] = 001b, Gain > 30 COMP[2:0] = 010b, Gain > 200 COMP[2:0] = 011b, Gain > 300 COMP[2:0] = 1xxb, Gain > 1 pA/ % % % ppm/°C NL GBW Non-Linearity Gain Bandwidth 3.3 8 24 80 240 0.8 900 70 400 37 490 16 680 8 195 4 130 1.5 89 0.8 1.6 3.8 6.5 9.3 0.17 4 100 ppm MHz BW −3 dB Bandwidth Gain = 10, COMP[2:0] = 000b Gain = 10, COMP[2:0] = 1xxb Gain = 20, COMP[2:0] = 000b Gain = 20, COMP[2:0] = 1xxb Gain = 50, COMP[2:0] = 001b Gain = 50, COMP[2:0] = 1xxb Gain = 100, COMP[2:0] = 010b Gain = 100, COMP[2:0] = 1xxb Gain = 200, COMP[2:0] = 010b Gain = 200, COMP[2:0] = 1xxb Gain = 500, COMP[2:0] = 011b Gain = 500, COMP[2:0] = 1xxb Gain = 1000, COMP[2:0] = 011b Gain = 1000, COMP[2:0] = 1xxb kHz SR Slew Rate (Note 7) COMP[2:0] = 000b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 001b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 010b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 011b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 1xxb, 10% to 90% of Step, VOUT = 2 VPP V/µs ts 0.01% Settling Time 2 V Step, CL = 10 pF, COMP[2:0] = 011b µs 3 www.national.com LMP8358 Symbol VOUT Parameter Output Voltage Swing High Conditions RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 RL > 1 MΩ to V+/2 Min Typ Max (Note 6) (Note 5) (Note 6) 32 40 12 17 7 12 28 38 12 17 8 13 21 15 32 25 28 37 1.8 1.9 0.014 85 5 0.02 10 100 1 10 100 2.1 2.2 1 15 Units mV from top rail Output Voltage Swing Low RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 RL > 1 MΩ to V+/2 mV from bottom rail IOUT Output Current Sourcing Output Current Sinking VOUT tied to V+/2 VOUT tied to V+/2 Fault detection off, VIN DIFF = 0V Fault detection on, VIN DIFF = 0V in Shutdown mA IS Supply Current mA mA µA µs mV V/V nA nA µA µA µA TSD_ON PSE ITEST Turn-on time from Shutdown Prescaler Error (Offset + Gain Error) Prescaler Gain Factor Fault Detection: Test Current Setting 1 (CUR[2:0] = 001b), VCM < V+ − 1.15V Setting 2 (CUR[2:0] = 010b), VCM < V+ − 1.15V Setting 3 (CUR[2:0] = 011b), VCM < V+ − 1.15V Setting 4 (CUR[2:0] = 100b), VCM < V+ − 1.15V Setting 5 (CUR[2:0] = 101b), VCM < V+ − 1.15V VCM = V+/2 5.0V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 5.0V , V− = 0V, VREF = V+/2, VCM = V+/2, RL = 10 kΩ to VREF, CL = 10 pF; Serial Control Register: G[2:0] = 110b (Gain = 1000x), COMP[2:0] = 000b, MUX[1:0] = 00b, POL, SHDN, FILT, PIN = 0b, CUR[2:0] = 000b. Boldface limits apply at the temperature extremes. Symbol VOS Parameter Input Offset Voltage VCM = V+/2 VCM = 0V TCVOS CMRR Input Offset Voltage Temperature Drift VCM = V+/2 (Note 8) VCM = 0V Common Mode Rejection Ratio V− − 0.1V ≤ VCM ≤ V+ − 1.4V V− + 0.1V ≤ VREFF ≤ V+ − 1.4V CMRR ≥ 115 dB 116 105 115 105 −0.1 142 150 3.6 Conditions Min Typ Max (Note 6) (Note 5) (Note 6) 0.9 0.9 10 15 10 15 50 50 Units µV nV/°C dB dB V VREFRR VREF Rejection Ratio CMVR Common Mode Voltage Range www.national.com 4 LMP8358 Symbol PSRR EMIRR ZINDM ZINCM VINDM IB IOS en Parameter Power supply Rejection Ratio Electro Magnetic Interference Rejection Ratio Differential Input Impedance Common Mode Input Impedance Differential Mode Input Voltage Input Bias Current Input Offset Current Input Voltage Noise Density Conditions 2.7V ≤ V+ ≤ 5.5V +IN / −IN, VRF = 100 mVP, f = 900 MHz +IN / −IN, VRF = 100 mVP, f = 1800 MHz Min Typ Max (Note 6) (Note 5) (Note 6) 112 105 138 83 93 50||1 50||1 ±100 0.006 0.2 1.2 2 113 Units dB dB MΩ ‖ pF MΩ ‖ pF mV nA pA Gain = 10, f = 1 kHz Gain = 20, f = 1 kHz Gain = 50, f = 1 kHz Gain = 100, f = 1 kHz Gain = 200, f = 1 kHz Gain = 500, f = 1 kHz Gain = 1000, f = 1 kHz Gain = External, f = 1 kHz 25 28 26 25 28 26 25 25 0.7 0.7 0.5 0.6 0.6 0.5 0.6 0.6 0.5 0.03 0.03 0.03 3 16 0.1 0.15 0.15 0.25 pA/ % % % ppm/°C µVPP nV/ en Input Voltage Noise Gain = 10, 0.1 Hz < f < 10 Hz Gain = 20, 0.1 Hz < f < 10 Hz Gain = 50, 0.1 Hz < f < 10 Hz Gain = 100, 0.1 Hz < f < 10 Hz Gain = 200, 0.1 Hz < f < 10 Hz Gain = 500, 0.1 Hz < f < 10 Hz Gain = 1000, 0.1 Hz < f < 10 Hz Gain = External, 0.1 Hz < f < 10 Hz In GE GE GE Input Current Noise Density Gain Error Gain Error Gain Error Contribution from chip Gain Error Temperature Coefficient Gain = 100, f = 1 kHz Gain = 10, 20, 50, 100, 200, 500 VOUT = VREF + 1V and VOUT = VREF − 1V Gain = 1000 VOUT = VREF + 1V and VOUT = VREF − 1V VOUT = VREF + 1V and VOUT = VREF − 1V For all gain settings (internal and external), VOUT = VREF + 1V and VOUT = VREF − 1V COMP[2:0] = 000b, Gain > 10 COMP[2:0] = 001b, Gain > 100 COMP[2:0] = 010b, Gain > 200 COMP[2:0] = 011b, Gain > 500 COMP[2:0] = 1xxb, Gain => 1 NL GBW Non-Linearity Gain Bandwidth 3 8 24 80 240 0.8 100 ppm MHz 5 www.national.com LMP8358 Symbol BW Parameter −3 dB Bandwidth Conditions Gain = 10, COMP[2:0] = 000b Gain = 10, COMP[2:0] = 1xxb Gain = 20, COMP[2:0] = 000b Gain = 20, COMP[2:0] = 1xxb Gain = 50, COMP[2:0] = 001b Gain = 50, COMP[2:0] = 1xxb Gain = 100, COMP[2:0] = 010b Gain = 100, COMP[2:0] = 1xxb Gain = 200, COMP[2:0] = 010b Gain = 200, COMP[2:0] = 1xxb Gain = 500, COMP[2:0] = 011b Gain = 500, COMP[2:0] = 1xxb Gain = 1000, COMP[2:0] = 011b Gain = 1000, COMP[2:0] = 1xxb Min Typ Max (Note 6) (Note 5) (Note 6) 930 74 385 37 460 16 640 8 195 4 130 1.5 89 0.8 1.7 5.0 9.0 11.0 0.16 4 52 62 22 30 12 17 42 55 16 22 12 17 23 16 34 30 31 41 1.8 1.9 0.006 85 5 0.02 2.1 2.2 1 8 Units kHz SR Slew Rate (Note 7) COMP[2:0] = 000b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 001b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 010b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 011b, 10% to 90% of Step, VOUT = 2 VPP COMP[2:0] = 1xxb, 10% to 90% of Step, VOUT = 2 VPP V/µs ts VOUT 0.01% Settling Time Output Voltage Swing High 2 V Step, CL = 10 pF, COMP[2:0] = 011b RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 RL > 1 MΩ to V+/2 µs mV from top rail Output Voltage Swing Low RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 RL > 1 MΩ to V+/2 mV from bottom rail IOUT Output Current Sourcing Output Current Sinking VOUT tied to V+/2 VOUT tied to V+/2 Fault detection off, VIN DIFF = 0V Fault detection on, VIN DIFF = 0V in Shutdown mA IS Supply Current mA mA µA µs mV V/V TSD_ON PSE Turn-on time from Shutdown Prescaler Error (Offset + Gain Error) Prescaler Gain Factor VCM = V+/2 www.national.com 6 LMP8358 Symbol ITEST Parameter Fault Detection: Test Current Conditions Setting 1 (CUR[2:0] = 001b), VCM < V+ − 2.25V Setting 2 (CUR[2:0] = 010b), VCM < V+ − 2.25V Setting 3 (CUR[2:0] = 011b), VCM < V+ − 2.25V Setting 4 (CUR[2:0] = 100b), VCM < V+ − 2.25V Setting 5 (CUR[2:0] = 101b), VCM < V+ − 2.25V Min Typ Max (Note 6) (Note 5) (Note 6) 10 100 1 10 100 Units nA nA µA µA µA Electrical Characteristics (Serial Interface) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ − V− ≥ 2.7V, V+ ≥ VHSER/VLPAR, V− ≤ VLSER/VHPAR, VD = (VHSER/VLPAR) − (VLSER/VHPAR) ≥ 2.5V. Symbol VIL VIH VOL VOH ISDO Parameter Input Logic Low Threshold Input Logic High Threshold Output Logic Low Threshold Output Logic High Threshold Output Source Current, SDO Output Sink Current, SDO IOZ t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 tr/tf Output Tri-state Leakage Current, SDO High Period, SCK Low Period, SCK Set Up Time, CSB to SCK Set Up Time, SDI to SCK Hold Time, SCK to SDI Prop. Delay, SCK to SDO ISDO = 2mA ISDO = 2mA VD = 3.3V or 5.0V, CSB = 0V, VOH = V+ – 0.7V VD = 3.3V or 5.0V, CSB = 0V, VOL = 1.0V VD = 3.3V or 5.0V, CSB = VD = 3.3V or 5V (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) 50 50 50 50 10 1.5 5 100 100 50 30 10 60 VD − 0.2V −2 2 ±1 mA 0.7 × VD 0.2 Conditions Min (Note 6) Typ (Note 5) Max (Note 6) 0.3 × VD Units V V V µA ns ns ns ns ns ns ns ns ns ns ns ns Hold Time, SCK Transition to CSB (Note 9) Rising Edge CSB Inactive Prop. Delay, CSB to SDO Active (Note 9) (Note 9) Prop. Delay, CSB to SDO Inactive (Note 9) Hold Time, SCK Transition to CSB (Note 9) Falling Edge Signal Rise and Fall Times (Note 9) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but for which specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics. Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22–A115–A (ESD MM std. of JEDEC). FieldInduced Charge-Device Model, applicable std. JESD22–C101–C (ESD FICDM std. of JEDEC). Note 3: The short circuit test is a momentary test which applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can exceed the maximum allowable junction temperature of 150°C. Note 4: The maximum power dissipation is a function of TJ(MAX), θJA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/ θJA. All numbers apply for packages soldered directly onto a PC Board. Note 5: Typical Values indicate the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material. Note 6: All limits are guaranteed by testing or statistical analysis. 7 www.national.com LMP8358 Note 7: Slew rate is the average of the rising and falling slew rates. Note 8: The offset voltage average drift is determined by dividing the value of VOS at the temperature extremes by the total temperature change. Note 9: Load for these tests is shown in the Timing Diagram Test Circuit. Connection Diagram 14-Pin SOIC/ 14-Pin TSSOP 30045402 Top View Pin Descriptions Pin Name Serial +IN −IN REFS REFF FB OUT V+ VHSER/VLPAR V− VLSER/VHPAR CSB/SHDN SCK/G2 SDI/G1 SDO/G0 Set Low Chip Select Serial Clock Serial Data In Serial Data Out Set High Negative Supply Set High Shutdown (Active High) Gain (MSB) Gain Gain (LSB) Positive Input Negative Input Reference Sense Reference Force Feedback Output Positive Supply Set Low Communication Mode Parallel Ordering Information Package 14-Pin SOIC 14-Pin TSSOP Part Number LMP8358MA LMP8358MAX LMP8358MT LMP8358MTX Package Marking LMP8358MA LMP8358MT Transport Media 55 Units/Rail 2.5k Units Tape and Reel 94 Units/Rail 2.5k Units Tape and Reel NSC Drawing M14A MTC14 www.national.com 8 LMP8358 Block Diagram 30045406 14-Pin SOIC/ 14-Pin TSSOP 9 www.national.com LMP8358 Timing Diagrams 30045403 SPI Timing Diagram 30045453 Timing Diagram Test Circuit www.national.com 10 LMP8358 Typical Performance Characteristics Gain vs. Frequency for Various COMP Settings V+ = 3.3V and TA = 25°C unless otherwise noted. Gain vs. Frequency for Various COMP Settings 300454c2 300454c3 Gain vs. Frequency for Various COMP Settings Gain vs. Frequency for Various Cap Loads 300454c4 300454c5 Gain vs. Frequency for Various Cap Loads Gain Error vs. Common Mode Voltage, VS = 5V 300454c6 300454f4 11 www.national.com LMP8358 Gain Error vs. Common Mode Voltage, VS = 3.3V Gain Error Distribution, Gain = 10, VS = 3.3V 300454g4 300454f5 Gain Error Distribution, Gain = 100, VS = 3.3V Gain Error Distribution, Gain = 1000, VS = 3.3V 300454g5 300454g6 VOS Distribution, VS = 3.3V VOS Distribution, VS = 5.0V 300454g2 300454g3 www.national.com 12 LMP8358 TCVOS Distribution, VS = 3.3V TCVOS Distribution, VS = 5V 300454g7 300454g8 VOS vs. VCM, VS = 3.3V VOS vs. VCM, VS = 3.3V 300454e5 300454e6 VOS vs. VCM, VS = 5.0V VOS vs. VCM, VS = 5.0V 300454e7 300454e8 13 www.national.com LMP8358 VOS vs. VREF, VS = 3.3V CMRR vs. Frequency 300454g9 300454d3 CMRR vs. Frequency PSRR vs. Frequency 300454d8 300454d9 Voltage Noise vs. Time Voltage Noise vs. Frequency 300454d2 300454c9 www.national.com 14 LMP8358 Small Signal Step Response for Various COMP Settings Small Signal Step Response for Various COMP Settings 300454d4 300454d5 Large Signal Step Response for Various COMP Settings Large Signal Step Response for Various COMP Settings 300454d6 300454d7 Positive Overshoot vs. CLOAD Supply Current vs. Supply Voltage 300454e3 300454e9 15 www.national.com LMP8358 Input Bias Current vs. VCM, VS = 3.3V Input Bias Current vs. VCM, VS = 5.0V 300454f0 300454f1 THD+N vs. Frequency THD+N vs. VOUT 300454e0 300454e2 ITEST1 vs. VCM ITEST2 vs. VCM 300454g0 300454f9 www.national.com 16 LMP8358 ITEST3 vs. VCM ITEST4 vs. VCM 300454f8 300454f7 ITEST5 vs. VCM Output Swing High vs. Supply Voltage 300454f2 300454f6 Output Swing Low vs. Supply Voltage EMIRR IN+ vs. Frequency 300454f3 300454e1 17 www.national.com LMP8358 Application Information INTRODUCTION The LMP8358 is a precision programmable gain instrumentation amplifier. Its gain can be programmed to 10, 20, 50, 100, 200, 500 or 1000 through an SPI-compatible serial interface or through a parallel interface. Alternatively, its gain can be set to an arbitrary value using external resistors. Note that at low gains the dynamic range is limited by the maximum input differential voltage of ±100mV. The LMP8358 uses patented techniques to measure and continuously correct its input offset voltage, eliminating offset drift over time and temperature, and the effect of 1/f noise. Its ground sensing CMOS input features a high CMRR and low input bias currents. It is capable of sensing differential input voltages in a commonmode range that extends from 100 mV below the negative supply to 1.4V below the positive supply, making it an ideal solution for interfacing with ground-referenced sensors, supply-referenced sensor bridges, and any other application requiring precision and long term stability. Additionally, the LMP8358 includes fault detection circuitry, so open and shorted inputs can be detected, as well a deteriorating connection to the signal source. Other features that make the LMP8358 a versatile solution for many applications are: its rail-to-rail output, low input voltage noise and high gain-bandwidth product. TRANSIENT RESPONSE TO FAST INPUTS The LMP8358 is a current-feedback instrumentation amplifier that consists of two auto-zeroed input stages. These two input stages are operated in a ping-pong fashion: as one stage is auto-zeroed the other stage provides the path between the input pins and the output. The auto-zeroing decreases offset, offset drift, and 1/f noise while the ping-pong architecture provides a continuous path between the input and the output. As with all devices that use auto-zeroing, care must be taken with the signal frequency used with the device. On-chip continuous auto-zero correction circuitry eliminates the 1/f noise and significantly reduces the offset voltage and offset voltage drift; all of which are very low-frequency events. For slow-changing sensor signals, below 2kHz, this correction is transparent. Higher-frequency signals as well as fast changing edges will show a settling and ramping time lasting about 1μs. Like all auto-zeroing devices, if the input frequency is above the autozero frequency, aliasing will occur. This can occur both at the auto-zeroing frequency of about 12kHz and the ping-pong frequency of about 50kHz. If needed, a low-pass filter should be placed on the output of the LMP8358 to filter out this disturbance. COMMUNICATION WITH THE PART AND REGISTER DESCRIPTION The LMP8358 supports a serial and a parallel digital interface mode as shown in Figure 1. Parallel user mode Gain is set using G0, G1 and G2 pins. The shutdown mode can be activated by asserting SHDN. Fault detection features are unavailable. Serial user mode The part is SPI-programmable through SDI, SCK, SDO and CSB. All features are available. 30045410 www.national.com 18 LMP8358 300454a5 FIGURE 1. (A) Communication with LMP8358 in Parallel Mode (B) Communication with LMP8358 in Serial Mode Communication Mode Selection The interface mode is determined by the two interface level pins VLSER/VHPAR and VHSER/VLPAR. VLSER/VHPAR < VHSER/ Serial Interface. VLSER= VLPAR Logic low level, VHSER = Logic high level. VLSER/VHPAR > VHSER/ Parallel interface. VLPAR = VLPAR Logic low level, VHPAR = Logic high level. The levels applied to the VLSER/VHPAR and VHSER/VLPAR pins should be between the V+ and V− levels as shown in Figure 2. Table 1. Function of Digital IO Pins, Parallel Mode Pin Name G0 G1 G2 SHDN VHPAR VLPAR Description Gain setting (LSB) Gain setting Gain setting (MSB) Shutdown (Active High) Positive logic level Negative logic level Table 2. Pin Levels for Setting Gain, Parallel Mode G2 G1 G0 Gain Setting Bandwidth Compensation Setting (Automatically Set) 930 kHz 385 kHz 460 kHz 640 kHz 195 kHz 130 kHz 89 kHz 800 kHz 000b 000b 001b 010b 010b 011b 011b 1xxb 0 0 0 30045409 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 10x (power-up default) 20x 50x 100x 200x 500x 1000x User defined 0 1 1 1 1 FIGURE 2. Communication Mode Selection. PARALLEL CONTROL INTERFACE MODE The LMP8358 is put into Parallel Mode by setting VLSER/ VHPAR > VHSER/VLPAR. The register in the LMP8358 does not control the settings of the LMP8358 in this mode. Gain and shutdown are set by placing a high or low logic level on pins 11 (SHDN), 12 (G2), 13 (G1), and 14 (G0), as shown in Table 1. Function of Digital IO Pins, Parallel Mode and Table 2. Pin Levels for Setting Gain, Parallel Mode. The logic high and low levels are defined by the voltages on the VLSER/ VHPAR and VHSER/VLPAR pins. See the START UP AND POWER ON RESET section for power on requirements when using the parallel mode. SERIAL CONTROL INTERFACE MODE The LMP8358 is put into Serial Mode by setting VLSER/VHPAR < VHSER/VLPAR. In the Serial Mode the LMP8358 can be programmed by using pins 11 – 14 as shown in Table 3. Function of Digital IO Pins, Serial Mode and the SPI Timing Diagram. The LMP8358 contains a 16 bit register which controls the performance of the part. These bits can be changed using the Serial Mode of communication. The register of the LMP8358 is shown in Table 4. LMP8358 Register Description, Serial Mode. Immediately after power on the register should be written with the value needed for the application. See the START UP AND POWER ON RESET section. 19 www.national.com LMP8358 Table 3. Function of Digital IO Pins, Serial Mode Pin Name SDO SDI SCK CSB VLSER VHSER Description Serial Data Out Serial Data In Serial Clock Chip Select Negative Logic level Positive Logic Level Bit No 11 12 13 14 15 Name PIN CUR0 CUR1 CUR2 N/A Description Fault detection pin selection Fault detection current setting (LSB) Fault detection current setting Fault detection current setting (MSB) Unused, set to 0 Table 4. LMP8358 Register Description, Serial Mode Bit No 0 1 2 3 4 5 6 7 8 9 10 Name G0 G1 G2 COMP0 COMP1 COMP2 MUX0 MUX1 POL SHDN FILT Description Gain setting (LSB) Gain setting Gain setting (MSB) Frequency compensation setting (LSB) Frequency compensation setting Frequency compensation setting (MSB) Input multiplexer selection (LSB) Input multiplexer selection (MSB) Input polarity switch Shutdown Enable Enable filtering using external cap Serial Control Interface Operation The LMP8358 gain, bandwidth compensation, shutdown, input options, and fault detection are controlled by an on board programmable register. Data to be written into the control register is first loaded into the LMP8358 via the serial interface. The serial interface employs an 16-bit double-buffered register for glitch-free transitions between settings. Data is loaded through the serial data input, SDI. Data passing through the shift register is output through the serial data output, SDO. The serial clock, SCK controls the serial loading process. All sixteen data bits are required to correctly program the amplifier. The falling edge of CSB enables the shift register to receive data. The SCK signal must be high during the falling and rising edge of CSB. Each data bit is clocked into the shift register on the rising edge of SCK. Data is transferred from the shift register to the holding register on the rising edge of CSB. Operation is shown in the SPI Timing Diagram. The serial control pins can be connected in one of two ways when two or more LMP8358s are used in an application. www.national.com 20 LMP8358 Star Configuration The configuration shown in Figure 3 can be used if each LMP8358 will always have the same value in each register. After the microcontroller writes, all registers will have the same value. Using multiple CSB lines as shown in Figure 4 allows different values to be written into each register. 300454a3 FIGURE 3. Star Configuration for Writing the Same Value Into Each Register 300454a4 FIGURE 4. Star Configuration for Writing Different Values Into Each Register Daisy Chain Configuration This configuration can be used to program the same or different values in the register of each LMP8358. The connections are shown in Figure 5. In this configuration the SDO pin of each LMP8358 is connected to the SDI pin of the following LMP8358. 300454a5 FIGURE 5. Daisy Chain Configuration 21 www.national.com LMP8358 The following two examples show how the registers are written in the Daisy Chain Configuration. Tabel 5. If all three LMP8358s need a gain of 100 with a compensation level of 010. (0000 0000 0001 0011) Register of LMP8358 #1 Register of LMP8358 #2 Register of LMP8358 #3 Notes Power on 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001 0011 0000 0000 0001 0011 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001 0011 Default power on state The data in the register of LMP8358 #1 is shifted into the register of LMP8358 #2, the data in the register of LMP8358 #2 is shifted into the register of LMP8358 #3. After first two bytes are 0000 0000 0001 0011 sent After second two bytes 0000 0000 0001 0011 are sent After third two bytes are sent 0000 0000 0001 0011 Table 6. If LMP8358 #1 needs a gain of 20 (0000 0000 0000 0001), LMP8358 #2 needs a gain of 1000 with a compensation level of 011 (0000 0000 0001 1110), and LMP8358 #3 needs a gain of 100 with a compenstation level of 010 (0000 0000 0001 0011). Register of LMP8358 #1 Register of LMP8358 #2 Register of LMP8358 #3 Notes Power on 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001 0011 0000 0000 0001 1110 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001 0011 Default power on state The data in the register of LMP8358 #1 is shifted into the register of LMP8358 #2, the data in the register of LMP8358 #2 is shifted into the register of LMP8358 #3 After first two bytes are 0000 0000 0001 0011 sent After second two bytes 0000 0000 0001 1110 are sent After third two bytes are sent LMP8358 SETTINGS Gain (Serial, Parallel) When the LMP8358 is in Parallel Mode the gain can be set by applying a high or low level to pins 12 (G2), 13 (G1), and 14 (G0), as shown in Table 2. Pin Levels for Setting Gain, Parallel Mode. The Frequency Compensation bits are automatically set as shown in Table 2. Pin Levels for Setting Gain, Parallel Mode to optimize the bandwidth. In Serial Mode the gain is determined by setting G[2:0] as shown in Table 7. Gain Setting (Register bits 2:0) and the bandwidth can be changed using the Frequency Compensation bits in the register. Table 7. Gain Setting (Register bits 2:0) G2 0 0 0 0 1 1 1 1 G1 0 0 1 1 0 0 1 1 G0 0 1 0 1 0 1 0 1 Gain Setting 10x (power-up default) 20x 50x 100x 200x 500x 1000x User Defined 300454a6 0000 0000 0000 0001 When G[2:0] = 111b in either serial or parallel mode switch S1 is open and S2 is closed and the LMP8358 gain is set by external resistors as shown in Figure 6. The gain is: GAIN = 1 + (Z1/Z2) When the gain is set by external resistors and COMP[2:0] = 1xxb, a capacitor can be used to implement a noise reduction low pass filter. See the Filter and External Filter Capacitor (Serial) section. R1and CFILTER are placed between the OUT and FB pins. R2 is placed between the FB and REFS pins. When G[2:0] = 000b to 110b switch S1 is closed and switch S2 is open as shown in the Block Diagram. FIGURE 6. External Gain Set Resistors and Filter Capacitor www.national.com 22 LMP8358 Frequency Compensation (Serial) The gain-bandwidth compensation is set to one of five levels under program control. The amount of compensation can be decreased to maximize the available bandwidth as the gain of the amplifier is increased. The compensation level is selected by setting bits COMP[2:0] of the control register with 000b, 001b, 010b, 011b, or 1xxb. Table 8. Frequency Compensation (Register bits 5:3) shows the bandwidths achieved at the selectable gain and compensation settings. Note that for gains 10X and 20X, the recommended compensation setting is 000b. For the gain setting 50X, compensation settings may be 000b and 001b. Gain settings 100X and 200X may use the three bandwidth compensation settings 000b, 001b, and 010b. At gains of 500X and 1000X, all bandwidth compensation ranges may be used. Note that for lower gains, it is possible to under compensate the amplifier into instability. Table 8. Frequency Compensation (Register bits 5:3) Bandwidth Gain\COMP [2:0] 10 20 50 100 200 500 1000 User Defined Gain GBW Product 000 930 kHz 385 kHz 160 kHz 80 kHz 38 kHz 16 kHz 8 kHz > 10x 8 MHz 001 n/a n/a 460 kHz 225 kHz 95 kHz 40 kHz 22 kHz > 30x 24 MHz 010 n/a n/a n/a 640 kHz 195 kHz 85 kHz 50 kHz > 100x 80 MHz 011 n/a n/a n/a n/a n/a 130 kHz 89 kHz > 300x 240 MHz 1xx 74 kHz 37 kHz 16 kHz 8 kHz 4 kHz 1.5 kHz 0.8 kHz > 1x 0.8 MHz (For external filter cap) Input Multiplexer and Polarity Switch (Serial) The Input Multiplexer Selection bits MUX[1:0] and Polarity bit POL can be used to set the inputs of the LMP8358 to the states shown in Table 9. Input Multiplexer and Polarity (Register bits 8:6). Table 9. Input Multiplexer and Polarity (Register bits 8:6) MUX1 0 MUX0 0 Diff Input for POL = 0 Diff Input for POL = 1 0 1 1 0 1 1 Polarity Reversal When MUX[1:0] = 00b and POL = 0b the LMP8358 has the input of a normal instrumentation amplifier. The input for the LMP8358 is defined as Gain × (V+IN − V−IN). When POL = 1b, the input for the LMP8358 is defined as Gain × (V−IN – V+IN). Polarity reversal can be used to do system level calibration, 23 for example, to compensate for thermocouple voltages, residual offset of the LMP8358, or offsets of the sensor or ADC. Short Inputs When MUX[1:0] = 01b and POL = 0b both inputs are connected to the +IN pin of the LMP8358. The –IN pin is left www.national.com LMP8358 floating. When MUX[1:0] = 01b and POL = 1b both inputs are connected to the -IN pin of the LMP8358. The +IN pin is left floating. Compare Input to VWhen MUX[1:0] = 10b or 11b one external input of the LMP8358 is floating. The other external input is divided by 50 as shown in Table 9. Input Multiplexer and Polarity (Register bits 8:6). The internal instrumentation amplifier input that is not connected to the external pin is connected to V−. With a scale factor of 1/50 this gives an overall gain of 0.2x, 0.4x, 1x, 2x, 4x, 10x, or 20x depending on what the gain is set to with G[2:0] bits as shown in Table 10. Overall Gain using G[2:0], MUX[1:0] and POL. Table 10. Overall Gain using G[2:0], MUX[1:0] and POL G[2:0] 000b 001b 010b 011b 100b 101b 110b MUX[1:0] 10b or 11b 10b or 11b 10b or 11b 10b or 11b 10b or 11b 10b or 11b 10b or 11b Overall System Gain POL = 0b 0.2 0.4 1 2 4 10 20 POL = 1b −0.2 −0.4 −1 −2 −4 −10 −20 Table 13. RFILTER Value Gain 10 20 50 100 200 500 1000 User-Defined Gain RFILTER Value 18.5 kΩ 112 kΩ 168 kΩ 187 kΩ 1.12 MΩ 1.68 MΩ 1.87 MΩ External Resistor R1 The tolerance of the RFILTER value for the pre-defined gains is about ±3%. If an external filter cap is not used FILT should be set to 0b to prevent errors related to leakage currents on the FB pin. Fault Detection Pin and Current Setting (Serial) The LMP8358 has an internal current source that can be used to detect faults in the overall system. See the FAULT DETECTION METHODS Section. When PIN = 0b this current source is connected to the +IN pin. When PIN = 1b the current source is connected to the −IN pin. Table 14. Pin Current Source (Register bit 11) PIN 0 1 Current source is connected to +IN pin −IN pin Shutdown Enable (Serial, Parallel) When the SHDN bit of the LMP8358 register is set to 1b the part is put into shutdown mode. It will use less than 1μA in this state. Table 11. Shutdown (Register bit 9) SHDN 0 1 Mode Active mode Shutdown mode The Fault Detection Current bit, CUR[2:0] controls the amount of current that sent to the input pin as shown in Table 15. Fault Detection Current Source (Register bits 14:12). Table 15. Fault Detection Current Source (Register bits 14:12) CUR2 CUR1 CUR0 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 disconnected and powered down * 10 nA 100 nA 1 μA 10 µA 100 µA disconnected, but powered * Do Not Use Filter and External Filter Capacitor (Serial) The FILT bit controls the state of switch S2 shown in the Block Diagram. When G[2:0] = 000b to 110b, switch S2 will be open if FILT = 0b and S2 will be closed if FILT = 1b. When G[2:0] = 111b switch S2 is always closed and does not depend on the value in the FILT bit. When FILT = 1b and COMP[2:0] = 1xxb the LMP8358 is unitygain stable and an external filter cap can be applied as shown in Figure 6. The corner filter of the filter is: F-3dB = 1/(2πRFILTERCFILTER) RFILTER depends on the gain of the part and is shown inTable 13. RFILTER Value. Table 12. Filter (Register bit 10) FILT 0 1 Mode No external filter cap used External filter cap used * Leaving the fault detection current source powered allows it to switch between current levels faster, particularly when supplying currents less than 1 µA. www.national.com 24 LMP8358 FAULT DETECTION METHODS Using the Multiplexer, Polarity, and Current features the end user can detect faults in the system between the sensor and the LMP8358. These examples will use the set up shown in Figure 7 which shows a bridge sensor connected through some cabling to a supply and the LMP8358. The fault detection methods are described below. 300454b5 FIGURE 7. Bridge Connected to the LMP8358 With No Problems Common Mode Out of Range Figure 8 shows an example of a degraded connection between the bottom of the bridge and ground. This fault is shown by the 1.5 kΩ resistor placed between the bridge and ground. This will raise the common mode at the inputs of the LMP8358 to 4V, which is out of the CMVR. To determine the common mode voltage at the input pins, use the 1/50 feature by setting MUX[1:0] to 10b to test the +IN pin or to 11b to test the −IN pin, POL to 0b, and G[2:0] to 010b for a gain of 50 (0082x or 00C2x). This will give an overall gain of 1 and the output will read 4V for either MUX setting. 300454b6 FIGURE 8. Degraded Connection Between the Bottom of the Bridge and Ground 25 www.national.com LMP8358 Open Input Figure 9 shows an example of an open input fault. To sense this type of fault use the 1/50 feature by setting MUX[1:0] to 10b to test the +IN pin or to 11b to test the −IN pin, POL to 0b, PIN to 1b to test the −IN pin, and G[2:0] to 010b for a gain of 50, and inject 100µA current by setting CUR[2:0] = 101b (5082x or 58C2x). Since the input is open the input pin will be pulled to V+. With an overall gain of 1 the output will read 5V for open input. 300454b7 FIGURE 9. Open Input Input Shorted to V+ or V− Figure 10 shows an example of an input pin shorted to V+ or V−. To sense this fault, use the 1/50 feature by setting MUX [1:0] to 10b to test the +IN pin or to 11b to test the −IN pin, POL to 0b, and G[2:0] to 010b for a gain of 50 (0082x or 00C2x). This will give an overall gain of 1 and the output will read either V+ or V− depending on whether the input pin is shorted to V+ or V−. 300454b8 FIGURE 10. Input Shorted to V+ or V− www.national.com 26 LMP8358 Shorted Inputs Figure 11 shows the inputs of the LMP8358 shorted. To detect this fault set CUR[2:0] = 101b to inject a 100µA current and set the gain to 10× (5000x). The LMP8358 is set up with normal differential inputs. The output will read about 0.07V be- cause of the voltage drop across the internal ESD resistor, which has a value between 60Ω to 90Ω. If the gain is set to 100× with an injected current of 100µA the output will be about 0.7V. 300454b9 FIGURE 11. Shorted Inputs Degraded Input Line Figure 12 shows an example of a degraded connection between the bridge and the +IN pin of the LMP8358. This fault is shown by the 1 kΩ resistor placed between the bridge and the LMP8358. To detect this fault use the 1/50 feature by setting MUX[1:0] to 10b to test the +IN pin, POL to 0b, and G[2:0] to 010b for a gain of 50. This will give an overall gain of 1. Set CUR[2:0] = 101b to inject a 100µA current and read the output voltage (5082x). Next set MUX[1:0] to 11b and PIN to 1b to test the −IN pin as shown in Figure 13 and read the output (58C2x). If the voltages of these two measurements are different a degraded input fault exists. 300454c0 FIGURE 12. Degraded Input Line, Step 1 300454c1 FIGURE 13. Degraded Input Line, Step 2 27 www.national.com LMP8358 Fault Detection Example Using the fault detection features of the LMP8358 an end product, such as a scale, can periodically test that no damage has occurred to the system. A routine can be written that could, for example, run on start up, that will step through the fault detection features shown above and compare the output voltage to a table like that shown in Table 16. Fault Detection Matrix. If the circuit shown in Figure 7 is used the values shown in column 2 of Table 16. Fault Detection Matrix would show that the system is working correctly, the values in the columns under the Possible Faults heading would show that there is a potential problem and that operator attention is needed. Table 16. Fault Detection Matrix No Faults LMP8358 Register 00 82x 00 C2x 50 00x 50 03x 50 82x 58 C2x 50 82x 58 C2x VOUT 2.5V 2.5V 0.61V 4.97V 2.55V 2.55V 2.55V 2.55V VOUT VOUT < CMVR or VOUT > CMVR VOUT < CMVR or VOUT > CMVR V+ V+ 2.65V* 2.55V* 2.55V* 2.65V* Possible Cause Input is out of CMVR Input is out of CMVR +IN Open −IN Degraded +IN line Degraded +IN line Degraded −IN line Degraded −IN line Possible Faults VOUT V+ V+ 0.07V 0.7V Possible Cause VOUT Possible Cause +IN shorted to GND −IN shorted to GND +IN shorted to V+ 0V −IN shorted to V+ 0V Inputs shorted Inputs shorted * The values shown for a degraded input line will vary depending on the resistance in the line. This table uses the value in Figure 12 and Figure 13, 1kΩ. www.national.com 28 LMP8358 START UP AND POWER ON RESET During power on, 50µs after V+ − V− > 1V the LMP8358 resets the internal register to 0000x. If the digital supplies and inputs are undefined after the Power On Reset transients could occur which can cause erroneous data to be written over the default values in the register. The following should be done to prevent this from happening: • Bring all supplies up at the same time. All power supplies, analog and digital, should be brought up together within 40µs so that the supplies are not undefined after the Power On Reset at 50µs. This is easiest done by tying the VHSER/VLPAR and VLSER/VHPAR pins to the analog supplies. — Parallel Mode • Immediately after power on, write to the register the value needed for the application. (This is always recommended.) — Serial Mode LAYOUT The LMP8358 is a precision device that contains both analog and digital sections as shown in the Block Diagram. The PCB should be carefully designed to minimize the interaction between the analog and digital sections and to maximize the performance of the part. This should include the following: 0.1µF ceramic capacitors should be placed as close as possible to each supply pin. If a digital supply pin is tied to an analog pin only one 0.1µF capacitor is needed for both pins. A larger 1µF or 10µF capacitor should be located near the part for each supply. Digital and analog traces should be kept away from each other. Analog and digital traces should not run next to each other, if they do the digital signal can couple onto the analog line. The LMP8358 pinout is set up to simplify layout by not having analog, power, and digital pins mixed together. Pins 1 — 6 are the analog signals, pins 7 — 10 are the power pins, and pins 11 — 14 are the digital signals. Be aware of the signal and power return paths. The return paths of the analog, digital, and power sections should not cross each other and the return path should be underneath the respective signal or power path. The best PCB layout is if the bottom plane of the PCB is a solid plane. The REFF and REFS pins are connected to the bottom side of the gain resistors of the LMP8358 as shown in the Block Diagram. Any impedance on these pins will change the specified gain. If the REFF and REFS pins are to be connected to ground they should be tied directly to the ground plane and not through thin traces that can add impedance. If the REFF and REFS pins are to be connected to a voltage, the voltage source must be low impedance. This can be done by adding an op amp, such as the LMP7701, set up in a buffer configu- ration with the LMP7701 output connected to REFF, the negative input of the op amp connected to REFS, and the desired reference voltage connected to the positive input of the op amp as shown in Figure 14. DIFFERENTIAL BRIDGE SENSOR Non-amplified differential bridge sensors, which are used in a variety of applications, typically have a very small differential output signal. This small signal needs to be accurately amplified before it can be used by an ADC. The high DC performance of the LMP8358 makes it a good choice for use with a differential bridge sensor. This performance includes low input offset voltage, low input offset voltage drift, and high CMRR. The on chip EMI rejection filters available on the LMP8358 help remove the EMI interference introduced to the signal as shown in Figure 14 and improves the overall system performance. The circuit in Figure 14 shows a signal path solution for a typical bridge sensor using the LMP8358. The typical output voltage of a resistive load cell is 2mV/V. If the bridge sensor is using a 5V supply the maximum output voltage will be 2mV/ V × 5V = 10mV. The bridge voltage in this example is the same as the LMP8358 and ADC161S626 supply voltage of +5V. This 10mV signal must be accurately amplified by the LMP8358 to best match the dynamic range of the ADC. This is done by setting the gain of the LMP8358 to 200 which will give an output from the LMP8358 of 2V. To use the complete range of the ADC161S626 the VREF of the ADC should be set to half of the input or 1V. This is done by the resistor divider on the VREF pin of the ADC161S626. The negative input of the ADC and the REFF and REFS pins of the LMP8358 can be set to +2.5V to set the signal at the center of the supply. A resistor divider supplies +2.5V to the positive input of an LMP7701 set up in a buffer configuration. The LMP7701 acts as a low impedance source for the REFF pin. The VIOand VHSER/VLPAR pins should all be set to the same voltage as the microcontroller, +3.3V in this example. The VLSER/VHPAR pin should be connected to ground. The resistor and capacitor between the LMP8358 and the ADC161S626 serve a dual purpose. The capacitor is a charge reservoir for the sampling capacitor of the ADC. The resistor provides isolation for the LMP8358 from the capacitive load. The values listed in the ADC161S626 datasheet are 180Ω for the resistor and the 470pF for the capacitor. These two components also form a low pass filter of about 1.9MHz. If a filter is needed to attenuate disturbance from the internal auto−zeroing at 12kHz and the ping−pong frequency at 50kHz of the LMP8358 these values could be changed to 7870Ω and 0.01µF which will make a filter with a corner of about 2kHz. 29 www.national.com LMP8358 www.national.com 300454e4 30 FIGURE 14. Differential Bridge Sensor LMP8358 Physical Dimensions inches (millimeters) unless otherwise noted 14-Pin SOIC NS Package Number M14A 14–Pin TSSOP NS Package Number M14MT 31 www.national.com LMP8358 Zero-Drift, Programmable Instrumentation Amplifier with Diagnostics For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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