LMP8100AMAX

LMP8100 Programmable Gain Amplifier April 18, 2008 LMP8100 Programmable Gain Amplifier General Description The LMP8100 programmable gain amplifier features an adjustable gain from 1 to 16 V/V in 1 V/V increments. At the core of the LMP8100 is a precision, 33 MHz, CMOS input, rail-torail input/output operational amplifier with a typical open-loop gain of 110 dB. Amplifier closed-loop gain is set by an array of precision thin-film resistors. Amplifier control modes are programmed via a serial port that allows devices to be cascaded so that an array of LMP8100 amplifiers can be programmed by a single serial data stream. The control mode registers are double buffered to insure glitch-free transitions between programmed settings. The LMP8100 is part of the LMP® precision amplifier family and is ideal for a variety of applications. The amplifier features several programmable controls including: gain; a power-conserving shutdown mode which can reduce current consumption to only 20 μA; an input zeroing switch which allows the output offset voltage to be measured to facilitate system calibration; and four levels of internal frequency compensation which can be set to maximize bandwidth at the different gain settings. The LMP8100 comes in a 14-Pin SOIC package. Features Typical Values, TA = 25°C ■ Gain error (over temperature range) 0.03% — LMP8100A 0.075% — LMP8100 1 to 16 V/V in 1 V/V steps ■ Gain range ■ Programmable frequency compensation ■ Input zero calibration switch 250 μV ■ Input offset voltage (max, LMP8100A) 0.1 pA ■ Input bias current 12 nV/√Hz ■ Input noise voltage 33 MHz ■ Unity gain bandwidth 12 V/μs ■ Slew rate 20 mA ■ Output current 2.7V to 5.5V ■ Supply voltage range 5.3 mA ■ Supply current Rail-to-Rail output swing V+ −50 mV to V− +50 mV ■ Applications ■ ■ ■ ■ ■ ■ Industrial instrumentation Data acquisition systems Test equipment Scaling amplifier Gain control Sensor interface Simplified Block Diagram 20147607 LMP® is a registered trademark of National Semiconductor Corporation. © 2008 National Semiconductor Corporation 201476 www.national.com LMP8100 Block Diagram 20147602 www.national.com 2 LMP8100 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 VIN Differential Output Short Circuit Duration (Note 3) Supply Voltage (VS = V+ – V−) Voltage at Input and Output Pins Input Current Storage Temperature Range Junction Temperature (Note 4) 2 kV 200V 2.5V 6V V+ +0.3V, V− −0.3V ±10 mA −65°C to +150°C +150°C Soldering Information Lead Temperature, Infrared or Convection Reflow (20 sec) Lead Temperature, Wave Solder (10 sec) 235°C 260°C Operating Ratings (Note 1) 2.7V to 5.5V −40°C to +125°C −40°C to +85°C 145°C/W Supply Voltage (VS = V+ – V−) Junction Temperature Range (Note 4) LMP8100A LMP8100 Package Thermal Resistance (θJA (Note 4) 14-Pin SOIC 5V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 5V , V– = 0V, DGND = 0V, +IN = GRT = V+/2, RL = 10 kΩ to V+/2; Gain = 1 V/V. Boldface limits apply at the temperature extremes. Symbol Gain Error Gain Error Parameter LMP8100A LMP8100 Conditions 0V < +IN (DC) < 3.5V, 1 V/V ≤ Gain ≤ 16 V/V, 0.5V < VOUT < 4.5V +IN > 3.5V, 0.3V < VOUT < 4.7V Gain = 16 Gain = 16 0.5 0.8 ±50 ±50 (Note 8) 1.5 0.1 f = 10 kHz, 1 V/V ≤ Gain ≤ 16 V/V f = 0.1 Hz to 10 Hz, 1 V/V ≤ Gain 12 3.8 33 15.5 9.5 12 90 85 4.9 4.85 100 4.95 50 15 –0.2 0 5.3 20 5.2 5.0 6.0 7.2 100 150 V/µs dB V mV mA V mA MHz Min (Note 6) Typ (Note 5) 0.015 0.015 Max (Note 6) 0.03 0.03 0.075 0.075 0.1 0.2 2.2 4.8 ±250 ±450 ±400 ±600 5 5 100 Units % LMP8100 Gain Error (Gain = 1 V/V) Extended +IN Range TCGE VOS Gain Drift Input Offset Voltage LMP8100A LMP8100 LMP8100A LMP8100 TCVOS IB en Input Offset Temp Coefficient Input Bias Current Input-Referred Noise Voltage % ppm/°C µV µV/°C pA nV/ µVPP ≤ 16 V/V BW Bandwidth C1 = C0 = 0, Gain = 1 V/V C1 = C0 = 0, Gain = 2 V/V C1 = C0 = 1, Gain = 16 V/V SR PSRR VO Slew Rate Power Supply Rejection Ratio Output Swing High Output Swing Low (Note 7) 2.7V < V+ < 5.5V +IN = 5V +IN = 0V Sourcing and Sinking IO VIN IS Output Current Input Voltage Range Supply Current 3 www.national.com LMP8100 Symbol IPD Parameter Supply Current, Power Down Feedback Resistance Conditions Min (Note 6) Typ (Note 5) 3.5 5.6 Max (Note 6) 20 40 Units µA kΩ GΩ RIN Input Impedance f = 10 Hz >10 3.3V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 3.3V , V– = 0V, DGND = 0V, +IN = GRT = V+/2, RL = 10 kΩ to V+/2; Gain = 1 V/V. Boldface limits apply at the temperature extremes. Symbol Gain Error Gain Error Parameter LMP8100A LMP8100 Conditions 0V < +IN < 1.8V, 1 V/V ≤ Gain ≤ 16 V/V, 0.3V < VOUT < 3.0V +IN > 1.8V, 0.3V < VOUT < 3.0V Gain = 16 Gain = 16 0.5 0.8 ±50 ±50 (Note 8) 1.5 0.1 f = 10 kHz, 1 V/V ≤ Gain ≤ 16 V/V f = 0.1 Hz to 10 Hz, 1 V/V ≤ Gain ≤ 16 V/V BW Bandwidth C1 = C0 = 0, Gain = 1 V/V C1 = C0 = 0, Gain = 2 V/V C1 = C0 = 1, Gain = 16 V/V SR PSRR VO Slew Rate Power Supply Rejection Ratio Output Swing High Output Swing Low IO VIN IS IPD Output Current Input Voltage Range Supply Current Supply Current, Power Down Feedback Resistance RIN Input Impedance (Note 7) 2.7V < V+ < 3.6V +IN = 3.3V +IN = 0V Sourcing and Sinking 15 −0.2 0 5.1 1.8 5.6 >10 90 80 3.2 3.15 12 3.8 33 15.5 9.5 12 100 3.25 50 20 3.5 3.3 5.8 7.0 20 40 100 150 V/µs dB V mV mA V mA µA kΩ GΩ MHz Min (Note 6) Typ (Note 5) 0.015 0.015 Max (Note 6) 0.03 0.03 0.075 0.075 0.1 0.2 2.2 4.8 ±250 ±450 ±400 ±600 5 5 100 Units % LMP8100 Gain Error (Gain = 1 V/V) Extended +IN Range TCGE VOS Gain Drift Input Offset Voltage LMP8100A LMP8100 LMP8100A LMP8100 TCVOS IB en Input Offset Temp Coefficient Input Bias Current Input-referred Noise Voltage % ppm/°C µV µV/°C pA nV/ µVPP www.national.com 4 LMP8100 Electrical Characteristics (Serial Interface) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ - V− ≥ 2.7V, VD = V+ - DGND ≥ 2.5V. Symbol VIL VIH ISDO Parameter Logic Low Threshold 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, CS to SCK Set Up Time, SDI to SCK Hold Time, SCK to SDI Prop. Delay, SCK to SDO Hold Time, SCK Transition to CS Rising Edge CS Inactive Prop. Delay, CS to SDO Active Prop. Delay, CS to SDO Inactive Hold Time, SCK Transition to CS Falling Edge Signal Rise and Fall Times VD = 3.3V or 5.0V, CS = 0V, VOH = V+ – 0.7V VD = 3.3V or 5.0V, CS = 0V, VOL = 1.0V VD = 3.3V or 5.0V, CS = VD = 3.3V or 5V (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) (Note 9) 10 1.5 5 50 50 50 50 100 100 50 30 10 60 0.7 × VD −7 10 ±1 mA Conditions Min (Note 6) Typ (Note 5) Max (Note 6) 0.3 × VD Units V V µA ns ns ns ns ns ns ns ns ns ns ns ns 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. 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. 5 www.national.com LMP8100 Connection Diagram 14-Pin SOIC 20147601 Top View Ordering Information Package Part Number LMP8100AMA 14-Pin SOIC LMP8100AMAX LMP8100MA LMP8100MAX Package Marking LMP8100AMA LMP8100MA Transport Media 55 Units/Rail 2.5k units Tape and Reel 55 Units/Rail 2.5k units Tape and Reel M14A NSC Drawing www.national.com 6 LMP8100 Timing Diagram Test Circuit 20147653 Timing Diagram 20147603 7 www.national.com LMP8100 Test Circuit Diagram 20147609 Test Circuit www.national.com 8 LMP8100 Typical Performance Characteristics Offset Voltage Distribution Offset Voltage Distribution 20147685 20147686 TCVOS Distribution TCVOS Distribution 20147683 20147684 VOS vs. VIN VOS vs. VIN 20147666 20147667 9 www.national.com LMP8100 VOS vs. VS VOS vs. VS 20147665 20147664 IB vs. VIN IB vs. VIN 20147662 20147663 IS vs. VS IS vs. VS 20147660 20147661 www.national.com 10 LMP8100 DC Gain Error A = 1 DC Gain Error A = 1 20147689 20147687 DC Gain Error A = 2 DC Gain Error A = 2 20147690 20147688 Small Signal Gain Error vs. +IN DC Level Small Signal Gain Error vs. +IN DC Level 20147633 20147651 11 www.national.com LMP8100 PSRR vs. Frequency PSRR vs. Frequency 20147610 20147611 IOUT vs. VOUT (Source) IOUT vs. VOUT (Sink) 20147669 20147668 IOUT vs. VOUT (Source) IOUT vs. VOUT (Sink) 20147671 20147670 www.national.com 12 LMP8100 Small Signal Step Response Small Signal Step Response 20147635 20147636 Small Signal Step Response Small Signal Step Response 20147637 20147638 Small Signal Step Response Small Signal Step Response 20147639 20147640 13 www.national.com LMP8100 Small Signal Step Response Small Signal Step Response 20147641 20147642 Large Signal Step Response Large Signal Step Response 20147643 20147644 Large Signal Step Response Large Signal Step Response 20147645 20147646 www.national.com 14 LMP8100 Large Signal Step Response Large Signal Step Response 20147647 20147648 Large Signal Step Response Large Signal Step Response 20147649 20147650 THD+N vs. Frequency THD+N vs. Frequency 20147675 20147674 15 www.national.com LMP8100 THD+N vs. VOUT THD+N vs. VOUT 20147673 20147672 Bandwidth vs. Capacitive Load Bandwidth vs. Capacitive Load 20147617 20147618 Peaking vs. Capacitive Load Peaking vs. Capacitive Load 20147621 20147622 www.national.com 16 LMP8100 Peaking vs. Capacitive Load Gain vs. Frequency 20147623 20147630 Gain vs. Frequency Gain vs. Frequency 20147629 20147628 Gain vs. Frequency AC Gain Error vs. Frequency 20147627 20147656 17 www.national.com LMP8100 AC Gain Error vs. Frequency AC Gain Error vs. Frequency 20147655 20147657 AC Gain Error vs. Frequency AC Gain Error vs. Frequency 20147658 20147659 Noise vs. Frequency 0.1 Hz to 10 Hz Noise 201476a2 20147654 www.national.com 18 LMP8100 Closed Loop Output Impedance vs. Frequency Input Impedance 20147631 201476a1 SDO V vs. I SDO V vs. I 20147634 20147652 19 www.national.com LMP8100 Applications Information LIFETIME DRIFT Offset voltage (VOS) and gain are electrical parameters which may drift over time. This drift, known as lifetime drift, is very common in operational amplifiers; however, its effect is more evident in precision amplifiers. This is due to the very low offset voltage specification and very precise gain specification of the LMP8100. The LMP8100 has an option to zero out the offset voltage. When the zero bit of the register is set high, +IN is connected to GRT. Output offset voltage can be measured and adjusted out of the signal path. See the “Input Zeroing” section, for more information. Numerous reliability tests have been performed to characterize this drift for the LMP8100. Prior to each long term reliability test the input offset voltage and gain at 2X and 16X of each LMP8100 was measured at room temperature. The long term reliability tests include Operating Life Time (OPL) performed at 150°C for an extended period of time and Temperature Humidity Bias Testing (THBT) at 85°C and 85% humidity for an extended period of time. The offset voltage and gain of 2X and 16X were measured again at room temperature after each reliability test. The offset voltage drift is the difference between the initial measurement and the later measurement, after the reliability test. The gain drift is the percentage difference between the initial measurement and the later measurement, after the reliability test. Figure 1 – Figure 4 show the offset voltage drift and gain drift after 1000 hours of OPL. Figure 5 – Figure 8 show the offset voltage drift and gain drift after 1000 hours of THBT. 20147692 FIGURE 2. OPL VOS Drift 20147693 FIGURE 3. OPL Gain Drift, A = 2 20147691 FIGURE 1. OPL VOS Drift 20147694 FIGURE 4. OPL Gain Drift, A = 16 www.national.com 20 LMP8100 20147695 20147698 FIGURE 5. THBT VOS Drift FIGURE 8. THBT Gain Drift, A = 16 POWER-ON RESET The LMP8100 has a power-up reset feature that sets all the register bits to 0 when the part is powered up. To implement this feature the CS and SCK pins must be held at or above VIH when the LMP8100 is powered-up. Failure to power up in this method can lead to an unpredictable state of the register bits after power-up. CONTROL REGISTER The control register retains the information which controls the amplifier gain, bandwidth compensation, input zeroing, and power down. The register is loaded by way of the serial control interface. The register is double buffered so that changes can be made with minimum effect on amplifier performance. Table 1 shows the organization of the control register. Table 2 gives the codes for gain setting, input zeroing and power down control. Table 3 shows the codes for the four gain-bandwidth compensation levels. TABLE 1. Control Register Format C1 MSB C0, C1: Compensation setting Zero: Zero Input PD: Power Down G0 to G3: Gain setting 20147696 FIGURE 6. THBT VOS Drift C0 Zero PD G3 G2 G1 G0 LSB 20147697 FIGURE 7. THBT Gain Drift, A = 2 21 www.national.com LMP8100 TABLE 2. Input Zero, Power-Down and Gain Setting Codes Zero PD 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 X 1 1 0 G3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 X X G2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 X X G1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 X X G0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 X X Non-Inverting Gain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Power Down Zero Input 16 16 16 16 11 11 11 6 6 2 V/V 1 TABLE 4. Amplifier Gain and Compensation vs. Bandwidth Gain dB 0 6.02 15.6 15.6 20.8 20.8 20.8 24.1 24.1 24.1 24.1 Compensation Bits C1 0 0 0 0 0 0 1 0 0 1 1 C0 0 0 0 1 0 1 0 0 1 0 1 33.0 15.5 4.2 8.3 2.0 3.9 6.6 1.3 2.3 3.8 9.5 3 dB Bandwidth (MHz) TABLE 3. Amplifier Gain Compensation Codes C1 C0 Compensation Level 0 1 2 3 Minimum Compensation Condition Maximum Compensation 0 0 1 1 0 1 0 1 NON-INVERTING AMPLIFIER OPERATION The principal application of the LMP8100 is as a non-inverting amplifier as shown in the simplified schematic, Figure 9. The amplifier supply voltage (V+ to V−) is specified as 5.5V maximum. The V– supply pin is connected to the system ground for single supply operation. V− can be returned to a negative voltage when required by the application. The digital supply voltage for the serial interface is applied between the V+ supply pin and DGND. AMPLIFIER GAIN SETTING AND BANDWIDTH COMPENSATION CONTROL The gain of the LMP8100 is set to one of 16 levels under program control by setting the appropriate bits G[3:0] of the control register with a number from 00h to 15h. This sets the gain to a level from 1 V/V to 16 V/V respectively. The gain-bandwidth compensation is also selectable to one of four 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 C[1:0] of the control register with a number from 00b to 11b with 00b being maximum compensation and 11b being minimum compensation. Table 4 shows the bandwidths achieved at several gain and compensation settings. It will be noted that for gains between X1 and X5, the recommended compensation setting is 00b. For gain settings between X6 and X10, compensation settings may be 00b and 01b. Gain settings between X11 and X15 may use the three bandwidth compensation settings between 00b and 10b. At a gain of X16, all bandwidth compensation ranges may be used. Note that, for lower gains, it is possible to undercompensate the amplifier into instability. 20147605 FIGURE 9. Basic Non-Inverting Amplifier www.national.com 22 LMP8100 GRT PINS The GRT pins must have a low impedance connection to either ground or a reference voltage. Any parasitical impedance on these pins will affect the gain accuracy of the LMP8100. Figure 10 shows a simplified schematic of the LMP8100 showing the internal gain resistors and an external parasitical resistance RP. The gain of the LMP8100 is determined by RF and RG, the values of which are set by the internal register. The gain of the amplifier is given by the equation traces to minimize the parasitical resistance. Figure 11 shows two suggested methods of connecting the GRT pins to a ground plane on the same layer or to a ground plane on a different layer of the PCB. Any resistance between the GRT pins and either ground or a reference voltage will change the gain to 20147676 FIGURE 11. GRT Connection Methods The GRT pin can be connected to a reference voltage source to provide an offset adjustment to the gain function. Any DC resistance that may be present between the voltage source and the GRT pin must be kept to an absolute minimum to avoid introducing gain errors into the circuit. INPUT ZEROING Measurements made with the LMP8100 in the signal path may be adjusted for the output offset voltage of the amplifier. For example: The measurement of VOUT for offset correction might be made using an ADC under microprocessor control. Output offset is measured under program control by setting the ZERO bit in the programming register. In this mode, +IN is disconnected from the input pin and internally connected to the GRT input. Figure 12 shows the LMP8100 in the input zeroing mode. 20147679 FIGURE 10. LMP8100 with External Parasitical Resistance The connection between the GRT pins and ground or a reference voltage should be as short as possible using wide 23 www.national.com LMP8100 20147606 FIGURE 12. Non-Inverting Input Zeroing Function SERIAL CONTROL INTERFACE OPERATION The LMP8100 gain, bandwidth compensation, power down, and input zeroing are controlled by data stored in a programming register. Data to be written into the control register is first loaded into the LMP8100 via the serial interface. The serial interface employs an 8-bit shift register. 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 eight data bits are required to correctly program the amplifier. The falling edge of CS enables the shift register to receive data. The SCK signal must be high during the falling and rising edge of CS. 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 CS. Operation is shown in the timing diagram,Figure 13. 20147624 FIGURE 13. Serial Control Interface Timing www.national.com 24 LMP8100 The serial control pins can be connected in one of two ways when two or more LMP8100s are used in an application. Star Configuration This configuration can be used if each LMP8100 will always have the same value in each register. The connections are shown in Figure 14. After the microcontroller writes a byte all registers will have the same value. 20147680 FIGURE 14. Star Configuration Daisy Chain Configuration lowing LMP8100. The following two examples show how the registers are written. This configuration can be used to program the same or different values in the register of each LMP8100. The connections are shown in Figure 15. In this configuration the SDO pin of each LMP8100 is connected to the SDI pin of the folIf all three LMP8100s need a gain of 11 with a compensation level of 10. (10001010) Register of LMP8100 #1 Power on Byte one sent Byte two sent Byte three sent 00000000 10001010 10001010 10001010 Register of LMP8100 #2 00000000 00000000 10001010 10001010 Register of LMP8100 #3 00000000 00000000 00000000 10001010 Notes Default power on state (see above) The data in the register of LMP8100 #1 is shifted into the register of LMP8100 #2, the data in the register of LMP8100 #2 is shifted into the register of LMP8100 #3. If LMP8100 #1 needs a gain of 11 with a compensation level of 10 (10001010), LMP8100 #2 needs a gain of 6 with a compensation of 00 (00000101), and LMP8100 #3 needs a gain of 2 with a compensation of 00 (00000001). Register of LMP8100 #1 Power on Byte one sent Byte two sent Byte three sent 00000000 00000001 00000101 10001010 Register of LMP8100 #2 00000000 00000000 00000001 00000101 Register of LMP8100 #3 00000000 00000000 00000000 00000001 Notes Default power on state (see above) The data in the register of LMP8100 #1 is shifted into the register of LMP8100 #2, the data in the register of LMP8100 #2 is shifted into the register of LMP8100 #3. 25 www.national.com LMP8100 20147681 FIGURE 15. Daisy Chain Configuration POWER SUPPLY PURITY AND BYPASSING Particular attention to power supply purity is needed in order to preserve the LMP8100's gain accuracy and low noise. The LMP8100 worst-case PSRR is 85 dB or 56.2 µV/V. Nevertheless, the usable dynamic range, gain accuracy and inherent low noise of the amplifier can be compromised through the introduction and amplification of power supply noise. To decouple the LMP8100 from supply line AC noise, a 0.1 µF capacitor should be located on each supply line, close to the LMP8100. Adding a 10 µF capacitor in parallel with the 0.1 µF capacitor will reduce the noise introduced to the LMP8100 even more by providing an AC path to ground for most frequency ranges. A power supply dropout (V+ -V− < 2.7V) can cause an unintended reset of the register. If a dropout occurs, the register will need to be reprogrammed with the correct values. SCALING AMPLIFIER The LMP8100 is ideally suited for use as an amplifier between a sensor that has a wide output range and an ADC. As the signal from the sensor changes the gain of the LMP8100 can be changed so that the entire input range of the ADC is being used at all times. Figure 16 shows a data acquisition system using the LMP8100 and the ADC121S101. The 100Ω resistor and 390 pF capacitor form an antialiasing filter for the ADC121S101. The capacitor also stores and delivers charge to the switched capacitor input of the ADC. The capacitive load on the LMP8100 created by the 390pF capacitor is decreased by the 100Ω resistor. 20147682 FIGURE 16. Data Acquisition System Using the LMP8100 www.national.com 26 LMP8100 BRIDGE AMPLIFIER In Figure 17 two LMP8100s are used with a LMP7711 to build an amplifier for the signal from a GMR Magnetic Field Sensor. The advantage of using the LMP8100 is that as the signal strength from the Magnetic Field Sensor decreases, the gain of the LMP8100 can be increased. An example is if the signal from this composite amplifier is used to drive an ADC. When the maximum magnetic field to be measured is applied, the gain of the LMP7711 can be set to supply a full range signal to the ADC input with the gain of the LMP8100 set to one. As the magnetic field decreases, the gain of the LMP8100 can be increased, so that the signal supplied to the ADC uses a maximum amount of the ADC input range. The following can be done to maximize performance: • Connect the GRT pins directly together and to GND with a low impedance trace. • Make the traces between the Magnetic Field Sensor and the LMP8100s short to minimize noise pickup. • Place 0.1 μF capacitors close to each of the LMP8100 supply pins. The following can be done to simplify the design: • Connect the SCL, SCK, and CS lines in parallel and one microcontroller can be used to drive both LMP8100s. • The SDO pin can be left floating. The LM317 and LM6171 are used for the supply of the Magnetic Sensor. The 100Ω potentiometer is used to adjust the supply voltage to the Magnetic Field Sensor. The 2 kΩ potentiometer is used to fine tune the negative supply to set the Magnetic Field Sensor output to zero. 20147625 FIGURE 17. Bridge Amplifier 27 www.national.com LMP8100 Physical Dimensions inches (millimeters) unless otherwise noted 14-Pin SOIC NS Package Number M14A www.national.com 28 LMP8100 Notes 29 www.national.com LMP8100 Programmable Gain Amplifier Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet 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/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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