LTC2422

LTC1966 Precision Micropower ∆∑ RMS-to-DC Converter FeaTures n n n DescripTion The LTC®1966 is a true RMS-to-DC converter that utilizes an innovative patented DS computational technique. The internal delta sigma circuitry of the LTC1966 makes it simpler to use, more accurate, lower power and dramatically more flexible than conventional log antilog RMS-to-DC converters. The LTC1966 accepts single-ended or differential input signals (for EMI/RFI rejection) and supports crest factors up to 4. Common mode input range is rail-to-rail. Differential input range is 1VPEAK, and offers unprecedented linearity. Unlike previously available RMS-to-DC converters, the superior linearity of the LTC1966 allows hassle free system calibration at any input voltage. The LTC1966 also has a rail-to-rail output with a separate output reference pin providing flexible level shifting. The LTC1966 operates on a single power supply from 2.7V to 5.5V or dual supplies up to ±5.5V. A low power shutdown mode reduces supply current to 0.5µA. The LTC1966 is insensitive to PC board soldering and stresses, as well as operating temperature. The LTC1966 is packaged in the space saving MSOP package which is ideal for portable applications. n n n n n n n n n Simple to Use, Requires One Capacitor True RMS DC Conversion Using DS Technology High Accuracy: 0.1% Gain Accuracy from 50Hz to 1kHz 0.25% Total Error from 50Hz to 1kHz High Linearity: 0.02% Linearity Allows Simple System Calibration Low Supply Current: 155µA Typ, 170µA Max Ultralow Shutdown Current: 0.1µA Constant Bandwidth: Independent of Input Voltage 800kHz –3dB, 6kHz ±1% Flexible Supplies: 2.7V to 5.5V Single Supply Up to ±5.5V Dual Supply Flexible Inputs: Differential or Single-Ended Rail-to-Rail Common Mode Voltage Range Up to 1VPEAK Differential Voltage Flexible Output: Rail-to-Rail Output Separate Output Reference Pin Allows Level Shifting Wide Temperature Range: –55°C to 125°C Small Size: Space Saving 8-Pin MSOP Package applicaTions n n True RMS Digital Multimeters and Panel Meters True RMS AC + DC Measurements L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and No Latency DS is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 6359576, 6362677, 6516291 and 6651036. LINEARITY ERROR (VOUT mV DC – VIN mV ACRMS) Typical applicaTion Single Supply RMS-to-DC Converter 2.7V TO 5.5V VDD DIFFERENTIAL INPUT 0.1µF OPT. AC COUPLING IN1 IN2 EN OUTPUT LTC1966 OUT RTN VSS GND CAVE 1µF 1966 TA01 Quantum Leap in Linearity Performance 0.2 0 LTC1966, ∆∑ –0.2 –0.4 –0.6 –0.8 –1.0 60Hz SINEWAVES 0 50 100 150 200 250 300 350 400 450 500 VIN (mV ACRMS) 1966 TA01b CONVENTIONAL LOG/ANTILOG + VOUT – 1966fb 1 LTC1966 absoluTe MaxiMuM raTings (Note 1) pin conFiguraTion Supply Voltage VDD to GND ............................................. – 0.3V to 7V VDD to VSS ............................................ – 0.3V to 12V VSS to GND ............................................. –7V to 0.3V Input Currents (Note 2) ...................................... ±10mA Output Current (Note 3) ..................................... ± 10mA ENABLE Voltage ....................... VSS – 0.3V to VSS + 12V OUT RTN Voltage ............................... VSS – 0.3V to VDD Operating Temperature Range (Note 4) LTC1966C/LTC1966I ............................– 40°C to 85°C LTC1966H .......................................... – 40°C to 125°C LTC1966MP ....................................... –55°C to 125°C Specified Temperature Range (Note 5) LTC1966C/LTC1966I ............................– 40°C to 85°C LTC1966H .......................................... – 40°C to 125°C LTC1966MP ....................................... –55°C to 125°C Maximum Junction Temperature ......................... 150°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C TOP VIEW GND IN1 IN2 VSS 1 2 3 4 8 7 6 5 ENABLE VDD OUT RTN VOUT MS8 PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 220°C/W orDer inForMaTion LEAD FREE FINISH LTC1966CMS8#PBF LTC1966IMS8#PBF LTC1966HMS8#PBF LTC1966MPMS8#PBF TAPE AND REEL LTC1966CMS8#TRPBF LTC1966IMS8#TRPBF LTC1966HMS8#TRPBF LTC1966MPMS8#TRPBF PART MARKING* LTTG LTTH LTTG LTTG PACKAGE DESCRIPTION 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP TEMPERATURE RANGE 0°C to 70°C –40°C to 85°C –40°C to 125°C –55°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ elecTrical characTerisTics SYMBOL PARAMETER Conversion Accuracy GERR VOOS LINERR Conversion Gain Error The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, VSS = – 5V, VOUTRTN = 0V, CAVE = 10µF VIN = 200mVRMS, , VENABLE = 0.5V unless otherwise noted. CONDITIONS 50Hz to 1kHz Input (Notes 6, 7) LTC1966C, LTC1966I LTC1966H, LTC1966MP (Notes 6, 7) LTC1966C, LTC1966I LTC1966H, LTC1966MP 50mV to 350mV (Notes 7, 8) MIN TYP ± 0.1 l l MAX ±0.3 ±0.4 ±0.7 0.2 0.4 0.6 0.15 UNITS % % % mV mV mV % Output Offset Voltage 0.1 l l l Linearity Error 0.02 1966fb 2 LTC1966 elecTrical characTerisTics SYMBOL PARAMETER PSRR Power Supply Rejection The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, VSS = – 5V, VOUTRTN = 0V, CAVE = 10µF VIN = 200mVRMS, , VENABLE = 0.5V unless otherwise noted. CONDITIONS (Note 9) LTC1966C, LTC1966I LTC1966H, LTC1966MP (Notes 6, 7, 10) l l l MIN TYP 0.02 MAX 0.15 0.20 0.3 0.8 1.0 2 30 VDD UNITS %V %V %V mV mV mV mV V MΩ MΩ VIOS Input Offset Voltage 0.02 Accuracy vs Crest Factor (CF) CF = 4 CF = 5 Input Characteristics IVR ZIN CMRRI VIMAX VIMIN PSRRI Input Voltage Range Input Impedance Input Common Mode Rejection Maximum Input Swing Minimum RMS Input Power Supply Rejection VDD Supply (Note 9) VSS Supply (Note 9) (Note 14) Average, Differential (Note 12) Average, Common Mode (Note 12) (Note 13) Accuracy = 1% (Note 14) l l l l l l 60Hz Fundamental, 200mVRMS (Note 11) 60Hz Fundamental, 200mVRMS (Note 11) l l –1 –20 VSS 8 100 7 1 1.05 200 5 µV/V V mV µV/V µV/V V kΩ kΩ µV/V V V 250 120 VSS 75 85 30 16 1.0 0.9 1.05 250 50 6 20 800 600 300 VDD 95 200 Output Characteristics OVR ZOUT CMRRO VOMAX PSRRO Output Voltage Range Output Impedance Output Common Mode Rejection Maximum Differential Output Swing Power Supply Rejection VENABLE = 0.5V (Note 12) VENABLE = 4.5V (Note 13) Accuracy = 2%, DC Input (Note 14) l l l l VDD Supply (Note 9) VSS Supply (Note 9) CAVE = 10µF CAVE = 10µF l l 1000 500 µV/V µV/V kHz kHz kHz Frequency Response f1P f10P f– 3dB VDD VSS IDD ISS IDDS ISSS IIH 1% Additional Error (Note 15) 10% Additional Error (Note 15) ±3dB Frequency (Note 15) Positive Supply Voltage Negative Supply Voltage Positive Supply Current Negative Supply Current Supply Currents Supply Currents ENABLE Pin Current High (Note 16) IN1 = 20mV, IN2 = 0V IN1 = 200mV, IN2 = 0V IN1 = 20mV, IN2 = 0V VENABLE = 4.5V VENABLE = 4.5V LTC1966H, LTC1966MP VENABLE = 4.5V l l l l Power Supplies 2.7 –5.5 155 158 12 0.5 –1 –2 –0.3 –0.1 –0.05 5.5 0 170 20 10 V V µA µA µA µA µA µA µA Shutdown Characteristics l l l l 1966fb 3 LTC1966 The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, VSS = – 5V, VOUTRTN = 0V, CAVE = 10µF VIN = 200mVRMS, , VENABLE = 0.5V unless otherwise noted. SYMBOL PARAMETER IIL VTH VHYS ENABLE Pin Current Low ENABLE Threshold Voltage CONDITIONS VENABLE = 0.5V LTC1966H, LTC1966MP VDD = 5V, VSS = –5V VDD = 5V, VSS = GND VDD = 2.7V, VSS = GND l l elecTrical characTerisTics MIN –2 –10 TYP –1 2.4 2.1 1.3 0.1 MAX –0.1 UNITS µA µA V V V V ENABLE Threshold Hysteresis Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The inputs (IN1, IN2) are protected by shunt diodes to VSS and VDD. If the inputs are driven beyond the rails, the current should be limited to less than 10mA. Note 3: The LTC1966 output (VOUT) is high impedance and can be overdriven, either sinking or sourcing current, to the limits stated. Note 4: The LTC1966C/LTC1966I are guaranteed functional over the operating temperature range of – 40°C to 85°C. The LTC1966H/ LTC1966MP are guaranteed functional over the operating temperature range of –55°C to 125°C. Note 5: The LTC1966C is guaranteed to meet specified performance from 0°C to 70°C. The LTC1966C is designed, characterized and expected to meet specified performance from –40°C to 85°C but is not tested nor QA sampled at these temperatures. The LTC1966I is guaranteed to meet specified performance from –40°C to 85°C. The LTC1966H is guaranteed to meet specified performance from –40°C to 125°C. The LTC1966MP is guaranteed to meet specified performance from –55°C to 125°C. Note 6: High speed automatic testing cannot be performed with CAVE = 10µF The LTC1966 is 100% tested with CAVE = 22nF Correlation . . tests have shown that the performance limits above can be guaranteed with the additional testing being performed to guarantee proper operation of all the internal circuitry. Note 7: High speed automatic testing cannot be performed with 60Hz inputs. The LTC1966 is 100% tested with DC and 10kHz input signals. Measurements with DC inputs from 50mV to 350mV are used to calculate the four parameters: GERR, VOOS, VIOS and linearity error. Correlation tests have shown that the performance limits above can be guaranteed with the additional testing being performed to guarantee proper operation of all internal circuitry. Note 8: The LTC1966 is inherently very linear. Unlike older log/antilog circuits, its behavior is the same with DC and AC inputs, and DC inputs are used for high speed testing. Note 9: The power supply rejections of the LTC1966 are measured with DC inputs from 50mV to 350mV. The change in accuracy from VDD = 2.7V to VDD = 5.5V with VSS = 0V is divided by 2.8V. The change in accuracy from VSS = 0V to VSS = – 5.5V with VDD = 5.5V is divided by 5.5V. Note 10: Previous generation RMS-to-DC converters required nonlinear input stages as well as a nonlinear core. Some parts specify a DC reversal error, combining the effects of input nonlinearity and input offset voltage. The LTC1966 behavior is simpler to characterize and the input offset voltage is the only significant source of DC reversal error. Note 11: High speed automatic testing cannot be performed with 60Hz inputs. The LTC1966 is 100% tested with DC stimulus. Correlation tests have shown that the performance limits above can be guaranteed with the additional testing being performed to verify proper operation of all internal circuitry. Note 12: The LTC1966 is a switched capacitor device and the input/ output impedance is an average impedance over many clock cycles. The input impedance will not necessarily lead to an attenuation of the input signal measured. Refer to the Applications Information section titled Input Impedance for more information. Note 13: The common mode rejection ratios of the LTC1966 are measured with DC inputs from 50mV to 350mV. The input CMRR is defined as the change in VIOS measured between input levels of VSS to VSS + 350mV and input levels of VDD – 350mV to VDD divided by VDD – VSS – 350mV. The output CMRR is defined as the change in VOOS measured with OUT RTN = VSS and OUT RTN = VDD – 350mV divided by VDD – VSS – 350mV. Note 14: Each input of the LTC1966 can withstand any voltage within the supply range. These inputs are protected with ESD diodes, so going beyond the supply voltages can damage the part if the absolute maximum current ratings are exceeded. Likewise for the output pins. The LTC1966 input and output voltage swings are limited by internal clipping. The maximum differential input of the LTC1966 (referred to as maximum input swing) is 1V. This applies to either input polarity, so it can be thought of as ±1V. Because the differential input voltage gets processed by the LTC1966 with gain, it is subject to internal clipping. Exceeding the 1V maximum can, depending on the input crest factor, impact the accuracy of the output voltage, but does not damage the part. Fortunately, the LTC1966’s ∆∑ topology is relatively tolerant of momentary internal clipping. The input clipping is tested with a crest factor of 2, while the output clipping is tested with a DC input. Note 15: The LTC1966 exploits oversampling and noise shaping to reduce the quantization noise of internal 1-bit analog-to-digital conversions. At higher input frequencies, increasingly large portions of this noise are aliased down to DC. Because the noise is shifted in frequency, it becomes a low frequency rumble and is only filtered at the expense of increasingly long settling times. The LTC1966 is inherently wideband, but the output accuracy is degraded by this aliased noise. These specifications apply with CAVE = 10µF and constitute a 3-sigma variation of the output rumble. Note 16: The LTC1966 can operate down to 2.7V single supply but cannot operate at ±2.7V. This additional constraint on VSS can be expressed mathematically as – 3 • (VDD – 2.7V) ≤ VSS ≤ Ground. 1966fb 4 LTC1966 Typical perForMance characTerisTics Gain and Offsets vs Input Common Mode 0.5 0.4 0.3 GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 –5 –4 –3 –2 –1 0 1 2 3 INPUT COMMON MODE (V) 4 5 GAIN ERROR VOOS VIOS VDD = 5V VSS = – 5V 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 GAIN ERROR Gain and Offsets vs Input Common Mode VDD = 5V VSS = GND 0.5 VIOS VOOS 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT COMMON MODE (V) 1966 G02 Gain and Offsets vs Input Common Mode 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 INPUT COMMON MODE (V) 1966 G01 VDD = 2.7V VSS = GND 1.0 0.8 VIOS GAIN ERROR 0.6 OFFSET VOLTAGE (mV) 0.4 0.2 0 VOOS –0.2 –0.4 –0.6 –0.8 –1.0 1966 G03 Gain and Offsets vs Output Common Mode 0.5 0.4 0.3 GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 –5 –4 –3 –2 –1 0 1 2 3 OUTPUT COMMON MODE (V) 4 5 VIOS GAIN ERROR VOOS VDD = 5V VSS = – 5V 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 Gain and Offsets vs Output Common Mode VDD = 5V VSS = GND VIOS VOOS GAIN ERROR 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 OUTPUT COMMON MODE (V) 1966 G05 Gain and Offsets vs Output Common Mode 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 OUTPUT COMMON MODE (V) 1966 G04 VDD = 2.7V VSS = GND 1.0 VIOS 0.8 0.6 OFFSET VOLTAGE (mV) 0.4 0.2 0 GAIN ERROR VOOS –0.2 –0.4 –0.6 –0.8 –1.0 1966 G06 Gain and Offsets vs Temperature 0.5 0.4 0.3 GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 VIOS GAIN ERROR VOOS VDD = 5V VSS = – 5V 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 Gain and Offsets vs Temperature VDD = 5V VSS = GND VIOS VOOS 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 GAIN ERROR –0.1 –0.2 –0.3 –0.4 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 Gain and Offsets vs Temperature VDD = 2.7V VSS = GND 1.0 0.8 VIOS GAIN ERROR 0.6 OFFSET VOLTAGE (mV) 0.4 0.2 0 VOOS –0.2 –0.4 –0.6 –0.8 –0.5 –0.5 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 1966 G09 –0.5 –0.5 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 1966 G08 –1.0 –0.5 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 1966 G07 1966fb 5 LTC1966 Typical perForMance characTerisTics Gain and Offsets vs VSS Supply 0.5 0.4 0.3 GAIN ERROR (%) 0.2 0.1 0 NOMINAL SPECIFIED CONDITIONS –0.1 –0.2 –0.3 –0.4 –0.5 –6 GAIN ERROR VOOS VIOS VDD = 5V 0.5 0.4 0.3 OFFSET VOLTAGE (mV) GAIN ERROR (%) 0.2 0.1 0 – 0.1 – 0.2 – 0.3 – 0.4 –4 –3 VSS (V) –2 –1 0 1966 G11 Gain and Offsets vs VDD Supply 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 2.5 3.0 3.5 4.0 VDD (V) 4.5 5.0 GAIN ERROR VOOS VSS = GND VIOS 1 0.8 OUTPUT VOLTAGE (mV DC) 0.6 OFFSET VOLTAGE (mV) 0.4 0.2 0 – 0.2 – 0.4 – 0.6 – 0.8 –1.0 5.5 1966 G10 Performance vs Crest Factor 201.0 200mVRMS SCR WAVEFORMS CAVE = 10µF 200.8 VDD = 5V O.1%/DIV 200.6 20Hz 200.4 100Hz 200.2 200.0 199.8 1.0 60Hz –5 –0.5 1.5 2.0 2.5 3.0 3.5 4.0 CREST FACTOR 4.5 5.0 1966 G15 Performance vs Large Crest Factors 230 220 OUTPUT VOLTAGE (mV DC) 210 200 190 180 170 200mVRMS SCR WAVEFORMS = 4.7µF C 160 VAVE= 5V DD 5%/DIV 150 6 2 3 5 4 1 CREST FACTOR 1966 G12 AC Linearity 0.20 VOUT (mV DC) – VIN (mV ACRMS) 0.15 0.10 0.05 0 60Hz SINEWAVES CAVE = 1µF VIN2 = GND 10 5 0 –5 –10 –15 –20 0 50 100 150 200 250 300 350 400 450 500 VIN1 (mV ACRMS) 1966 G13 Output Accuracy vs Signal Amplitude 1% ERROR AC INPUTS = 60Hz SINEWAVES VIN2 = GND FUNDAMENTAL FREQUENCY 20Hz 60Hz 100Hz 250Hz VOUT (mV DC) – VIN (mVRMS) –0.05 –0.10 –0.15 –0.20 –1% ERROR AC INPUT VDD = 5V DC INPUT VDD = 5V AC INPUT VDD = 3V 0 0.5 1 1.5 VIN1 (VRMS) 2 2.5 1966 G24 7 8 DC Linearity 0.10 CAVE = 1µF 0.08 VIN2 = GND 0.06 SUPPLY CURRENT (µA) VOUTDC – |VINDC| (mV) 0.04 0.02 0 –0.02 –0.04 –0.06 –0.08 –0.10 –500 –300 EFFECT OF OFFSETS MAY BE POSITIVE OR NEGATIVE 100 –100 VIN1 (mV) 300 500 1966 G14 Quiescent Supply Currents vs Supply Voltage 200 175 150 125 100 75 50 25 0 –25 0 1 ISS 2 5 3 4 VDD SUPPLY VOLTAGE (V) 6 1966 G16 Shutdown Currents vs ENABLE Voltage 250 200 SUPPLY CURRENT (µA) 150 100 50 0 ISS IEN IDD VDD = 5V VSS = GND IDD 500 250 0 –250 0 1 4 3 5 2 ENABLE PIN VOLTAGE (V) 6 1966 G18 ENABLE PIN CURRENT (nA) –50 –100 –500 1966fb 6 LTC1966 Typical perForMance characTerisTics Quiescent Supply Currents vs Temperature 170 160 150 140 IDD (µA) 130 120 110 100 VDD = 5V, VSS = GND VDD = 5V, VSS = – 5V VDD = 2.7V, VSS = GND VDD = 5V, VSS = – 5V VDD = 5V, VSS = GND VDD = 2.7V, VSS = GND 40 35 OUTPUT DC VOLTAGE (mV) 30 25 ISS (µA) 20 15 10 5 1 100 10K 100K 1K INPUT SIGNAL FREQUENCY (Hz) 1M 1966 G19 Input Signal Bandwidth 1000 0.1% ERROR 1% ERROR 10% ERROR –3dB OUTPUT DC VOLTAGE (mV) 202 200 198 196 194 192 190 188 186 Input Signal Bandwidth 100 10 0 90 – 60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 1966 G17 184 1%/DIV CAVE = 2.2µF 182 10 100 1 INPUT FREQUENCY (kHz) 1000 1966 G20 Bandwidth to 100kHz 202 201 OUTPUT DC VOLTAGE (mV) 200 VOUT (mV DC) 199 198 197 196 195 0 10 20 30 40 50 60 70 80 90 100 INPUT FREQUENCY (kHz) 1966 G21 DC Transfer Function Near Zero 30 25 20 15 10 5 0 –5 –10 –20 –15 –10 0 5 –5 VIN1 (mV DC) 10 15 20 COMMON MODE REJECTION RATIO (dB) VIN2 = GND THREE REPRESENTITIVE UNITS 110 100 90 80 70 60 50 40 30 20 Common Mode Rejection Ratio vs Frequency VDD = 5V VSS = –5V ±5V INPUT CONVERSION TO DC OUTPUT 0.5%/DIV CAVE = 47µF 10 100 1k 10k FREQUENCY (Hz) 100k 1M 1966 G23 1966 G22 1966fb 7 LTC1966 pin FuncTions GND (Pin 1): Ground. A power return pin. IN1 (Pin 2): Differential Input. DC coupled (polarity is irrelevant). IN2 (Pin 3): Differential Input. DC coupled (polarity is irrelevant). VSS (Pin 4): Negative Voltage Supply. GND to – 5.5V. VOUT (Pin 5): Output Voltage. This is high impedance. The RMS averaging is accomplished with a single shunt capacitor from this node to OUT RTN. The transfer function is given by: OUT RTN (Pin 6): Output Return. The output voltage is created relative to this pin. The VOUT and OUT RTN pins are not balanced and this pin should be tied to a low impedance, both AC and DC. Although it is typically tied to GND, it can be tied to any arbitrary voltage, VSS < OUT RTN < (VDD – Max Output). Best results are obtained when OUT RTN = GND. VDD (Pin 7): Positive Voltage Supply. 2.7V to 5.5V. ENABLE (Pin 8): An Active Low Enable Input. LTC1966 is debiased if open circuited or driven to VDD. For normal operation, pull to GND, a logic low or even VSS. ( VOUT – OUT RTN) = 2 Average (IN2 – IN1)      1966fb 8 LTC1966 applicaTions inForMaTion START READ RMS-TO-DC CONVERSION NOT SURE DO YOU NEED TRUE RMS-TO-DC CONVERSION? NO FIND SOMEONE WHO DOES AND GIVE THEM THIS DATA SHEET YES CONTACT LTC BY PHONE OR AT www.linear.com AND GET SOME NOW NO DO YOU HAVE ANY LTC1966s YET? YES DID YOU ALREADY TRY OUT THE LTC1966? NO DO YOU WANT TO KNOW HOW TO USE THE LTC1966 FIRST? YES YES NO READ THE TROUBLESHOOTING GUIDE. IF NECESSARY, CALL LTC FOR APPLICATIONS SUPPORT NO DID YOUR CIRCUIT WORK? READ THE DESIGN COOKBOOK YES CONTACT LTC AND PLACE YOUR ORDER YES NOW DOES YOUR RMS CIRCUIT WORK WELL ENOUGH THAT YOU ARE READY TO BUY THE LTC1966? NO READ THE TROUBLESHOOTING GUIDE AGAIN OR CALL LTC FOR APPLICATIONS SUPPORT 1966 TA02 1966fb 9 LTC1966 applicaTions inForMaTion RMS-TO-DC CONVERSION Definition of RMS RMS amplitude is the consistent, fair and standard way to measure and compare dynamic signals of all shapes and sizes. Simply stated, the RMS amplitude is the heating potential of a dynamic waveform. A 1VRMS AC waveform will generate the same heat in a resistive load as will 1V DC. 1V DC + – R The last two entries of Table 1 are chopped sine waves as is commonly created with thyristors such as SCRs and Triacs. Figure 2a shows a typical circuit and Figure 2b shows the resulting load voltage, switch voltage and load currents. The power delivered to the load depends on the firing angle, as well as any parasitic losses such as switch ON voltage drop. Real circuit waveforms will also typically have significant ringing at the switching transition, dependent on exact circuit parasitics. For the purposes of this data sheet, SCR waveforms refers to the ideal chopped sine wave, though the LTC1966 will do faithful RMS-to-DC conversion with real SCR waveforms as well. The case shown is for Θ = 90°, which corresponds to 50% of available power being delivered to the load. As noted in Table 1, when Θ = 114°, only 25% of the available power is being delivered to the load and the power drops quickly as Θ approaches 180°. With an average rectification scheme and the typical calibration to compensate for errors with sine waves, the RMS level of an input sine wave is properly reported; it is only with a nonsinusoidal waveform that errors occur. Because of this calibration, and the output reading in VRMS, the term true RMS got coined to denote the use of an actual RMS-to-DC converter as opposed to a calibrated average rectifier. + VLOAD – AC MAINS VLINE 1V ACRMS R SAME HEAT 1V (AC + DC) RMS R 1966 F01 Figure 1 Mathematically, RMS is the root of the mean of the square: VRMS = V2 Alternatives to RMS Other ways to quantify dynamic waveforms include peak detection and average rectification. In both cases, an average (DC) value results, but the value is only accurate at the one chosen waveform type for which it is calibrated, typically sine waves. The errors with average rectification are shown in Table 1. Peak detection is worse in all cases and is rarely used. Table 1. Errors with Average Rectification vs True RMS WAVEFORM Square Wave Sine Wave Triangle Wave SCR at 1/2 Power, Θ = 90° SCR at 1/4 Power, Θ = 114° VRMS 1.000 1.000 1.000 1.000 1.000 AVERAGE RECTIFIED (V) 1.000 0.900 0.866 0.637 0.536 + – VTHY + – ILOAD CONTROL 1966 F02a Figure 2a VLINE ERROR* 11% *Calibrate for 0% Error –3.8% –29.3% –40.4% Θ VLOAD VTHY ILOAD 1966 F02b Figure 2b 1966fb 10 LTC1966 applicaTions inForMaTion How an RMS-to-DC Converter Works Monolithic RMS-to-DC converters use an implicit computation to calculate the RMS value of an input signal. The fundamental building block is an analog multiply/ divide used as shown in Figure 3. Analysis of this topology is easy and starts by identifying the inputs and the output of the lowpass filter. The input to the LPF is the calculation from the multiplier/divider; (VIN)2/VOUT. The lowpass filter will take the average of this to create the output, mathematically:  ( V )2  IN = ,  VOUT    How the LTC1966 RMS-to-DC Converter Works The LTC1966 uses a completely new topology for RMSto-DC conversion, in which a ∆S modulator acts as the divider, and a simple polarity switch is used as the multiplier as shown in Figure 4. Dα VIN VOUT ∆–∑ REF VIN ±1 VOUT LPF VOUT Because VOUT is DC, 2  ( V )2   ( VIN )    IN , so  =  VOUT  VOUT   Figure 4. Topology of LTC1966 VOUT  ( V )2   IN  = , and VOUT ( VOUT )2 = ( VIN )2, or VOUT = ( VIN )2 = RMS( VIN ) (VIN ) 2 The ∆S modulator has a single-bit output whose average duty cycle (D) will be proportional to the ratio of the input signal divided by the output. The ∆S is a 2nd order modulator with excellent linearity. The single bit output is used to selectively buffer or invert the input signal. Again, this is a circuit with excellent linearity, because it operates at only two points: ±1 gain; the average effective multiplication over time will be on the straight line between these two points. The combination of these two elements again creates a lowpass filter input signal proportional to (VIN)2/VOUT, which, as shown above, results in RMS-to-DC conversion. The lowpass filter performs the averaging of the RMS function and must be a lower corner frequency than the lowest frequency of interest. For line frequency measurements, this filter is simply too large to implement on-chip, but the LTC1966 needs only one capacitor on the output to implement the lowpass filter. The user can select this capacitor depending on frequency range and settling time requirements, as will be covered in the Design Cookbook section to follow. This topology is inherently more stable and linear than log/antilog implementations primarily because all of the signal processing occurs in circuits with high gain op amps operating closed loop. VOUT VIN ×÷ LPF VOUT 1966 F03 Figure 3. RMS-to-DC Converter with Implicit Computation Unlike the prior generation RMS-to-DC converters, the LTC1966 computation does NOT use log/antilog circuits, which have all the same problems, and more, of log/antilog multipliers/dividers, i.e., linearity is poor, the bandwidth changes with the signal amplitude and the gain drifts with temperature. 1966fb 11 LTC1966 applicaTions inForMaTion More detail of the LTC1966 inner workings is shown in the Simplified Schematic towards the end of this data sheet. Note that the internal scalings are such that the ∆S output duty cycle is limited to 0% or 100% only when VIN exceeds ± 4 • VOUT. Linearity of an RMS-to-DC Converter Linearity may seem like an odd property for a device that implements a function that includes two very nonlinear processes: squaring and square rooting. However, an RMS-to-DC converter has a transfer function, RMS volts in to DC volts out, that should ideally have a 1:1 transfer function. To the extent that the input to output transfer function does not lie on a straight line, the part is nonlinear. A more complete look at linearity uses the simple model shown in Figure 5. Here an ideal RMS core is corrupted by both input circuitry and output circuitry that have imperfect transfer functions. As noted, input offset is introduced in the input circuitry, while output offset is introduced in the output circuitry. Any nonlinearity that occurs in the output circuity will corrupt the RMS in to DC out transfer function. A nonlinearity in the input circuitry will typically corrupt that transfer function far less, simply because with an AC input, the RMS-to-DC conversion will average the nonlinearity from a whole range of input values together. But the input nonlinearity will still cause problems in an RMS-to-DC converter because it will corrupt the accuracy as the input signal shape changes. Although an RMS-to-DC converter will convert any input waveform to a DC output, the accuracy is not necessarily as good for all waveforms as it is with sine waves. A common way to describe dynamic signal wave shapes is crest factor. The crest factor is the ratio of the peak value relative to the RMS value of a waveform. A signal with a crest factor of 4, for instance, has a peak that is four times its RMS value. Because this peak has energy (proportional to voltage squared) that is 16 times (42) the energy of the RMS value, the peak is necessarily present for at most 6.25% (1/16) of the time. The LTC1966 performs very well with crest factors of 4 or less and will respond with reduced accuracy to signals with higher crest factors. The high performance with crest factors less than 4 is directly attributable to the high linearity throughout the LTC1966. The LTC1966 does not require an input rectifier, as is common with traditional log/antilog RMS-to-DC converters. Thus, the LTC1966 has none of the nonlinearities that are introduced by rectification. The excellent linearity of the LTC1966 allows calibration to be highly effective at reducing system errors. See System Calibration section following the Design Cookbook. INPUT INPUT CIRCUITRY • VIOS • INPUT NONLINEARITY IDEAL RMS-TO-DC CONVERTER OUTPUT CIRCUITRY • VOOS • OUTPUT NONLINEARITY OUTPUT 1966 F05 Figure 5. Linearity Model of an RMS-to-DC Converter 1966fb 12 LTC1966 applicaTions inForMaTion DESIGN COOKBOOK The LTC1966 RMS-to-DC converter makes it easy to implement a rather quirky function. For many applications all that will be needed is a single capacitor for averaging, appropriate selection of the I/O connections and power supply bypassing. Of course, the LTC1966 also requires power. A wide variety of power supply configurations are shown in the Typical Applications section towards the end of this data sheet. Capacitor Value Selection The RMS or root-mean-squared value of a signal, the root of the mean of the square, cannot be computed without some averaging to obtain the mean function. The LTC1966 true RMS-to-DC converter utilizes a single capacitor on the output to do the low frequency averaging required for RMS-to-DC conversion. To give an accurate measure of a dynamic waveform, the averaging must take place over a sufficiently long interval to average, rather than track, the lowest frequency signals of interest. For a single averaging capacitor, the accuracy at low frequencies is depicted in Figure 6. Figure 6 depicts the so-called DC error that results at a given combination of input frequency and filter capacitor values1. It is appropriate for most applications, in which the output is fed to a circuit with an inherently band limited frequency response, such as a dual slope/integrating A/D converter, a ∆S A/D converter or even a mechanical analog meter. 0 –0.2 –0.4 –0.6 DC ERROR (%) –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 1 10 INPUT FREQUENCY (Hz) 20 50 60 100 1966 F06 However, if the output is examined on an oscilloscope with a very low frequency input, the incomplete averaging will be seen, and this ripple will be larger than the error depicted in Figure 6. Such an output is depicted in Figure 7. The ripple is at twice the frequency of the input because of the computation of the square of the input. The typical values shown, 5% peak ripple with 0.05% DC error, occur with CAVE = 1µF and fINPUT = 10Hz. If the application calls for the output of the LTC1966 to feed a sampling or Nyquist A/D converter (or other circuitry that will not average out this double frequency ripple) a larger averaging capacitor can be used. This trade-off is depicted in Figure 8. The peak ripple error can also be reduced by additional lowpass filtering after the LTC1966, but the simplest solution is to use a larger averaging capacitor. 1This frequency dependent error is in addition to the static errors that affect all readings and are therefore easy to trim or calibrate out. The Error Analyses section to follow discusses the effect of static error terms. ACTUAL OUTPUT WITH RIPPLE f = 2 × fINPUT PEAK RIPPLE (5%) IDEAL OUTPUT DC ERROR (0.05%) OUTPUT PEAK ERROR = DC ERROR + PEAK RIPPLE (5.05%) TIME DC AVERAGE OF ACTUAL OUTPUT 1966 F07 Figure 7. Output Ripple Exceeds DC Error C = 4.7µF C = 10µF C = 2.2µF C = 1.0µF C = 0.47µF C = 0.22µF C = 0.1µF Figure 6. DC Error vs Input Frequency 1966fb 13 LTC1966 applicaTions inForMaTion 0 –0.2 –0.4 PEAK ERROR (%) –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 1 10 INPUT FREQUENCY (Hz) 20 50 60 100 1966 F08 C = 100µF C = 2.2µF C = 1µF C = 47µF C = 22µF C = 10µF C = 4.7µF Figure 8. Peak Error vs Input Frequency with One Cap Averaging A 1µF capacitor is a good choice for many applications. The peak error at 50Hz/60Hz will be 10kHz inputs. – This is a fundamental characteristic of this topology. The LTC1966 is designed to work very well with inputs of 1kHz or less. It works okay as high as 1MHz, but it is limited by aliased ∆S noise. Solution: Bandwidth limit the input or digitally filter the resulting output. 8. Large errors occur at crest factors approaching, but less than 4. – Insufficient averaging. Solution: Increase CAVE. See Crest Factor and AC + DC Waveforms section for discussion of output droop. 9. Screwy results, errors > spec limits, typically 1% to 5%. – High impedance (85kΩ) and high accuracy (0.1%) require clean boards! Flux residue, finger grime, etc. all wreak havoc at this level. Solution: Wash the board. LTC1966 10. Gain is low by ≅ 1% or more, no other problems. – Probably due to circuit loading. With a DMM or a 10× scope probe, ZIN = 10MΩ. The LTC1966 output is 85kΩ, resulting in – 0.85% gain error. Output impedance is higher with the DC accurate post filter. Solution: Remove the shunt loading or buffer the output. – Loading can also be caused by cheap averaging capacitors. Solution: Use a high quality metal film capacitor for CAVE. LOADING DRAGS DOWN GAIN Helpful Hint: Sensitivity to leakages can be reduced significantly through the use of guard traces. KEEP BOARD CLEAN VOUT 5 6 10M mV 85k DCV OUT RTN DMM 200mVRMS IN –0.85% 1966 TS10 LTC1966 1966fb 33 LTC1966 Typical applicaTions ±5V Supplies, Differential, DC-Coupled RMS-to-DC Converter 5V VDD DC + AC INPUTS (1VPEAK DIFFERENTIAL) LTC1966 IN1 VOUT IN2 OUT RTN VSS GND EN –5V 1966 TA03 5V Single Supply, Differential, AC-Coupled RMS-to-DC Converter 5V VDD LTC1966 CAVE 1µF DC OUTPUT AC INPUTS (1VPEAK DIFFERENTIAL) CC 0.1µF IN1 VOUT IN2 OUT RTN VSS GND EN 1966 TA05 CAVE 1µF DC OUTPUT 2.7V Single Supply, Single Ended, AC-Coupled RMS-to-DC Converter with Shutdown 2.7V/3V CMOS OFF ON 2.7V 2V EN VDD AC INPUT (1VPEAK) CC 0.1µF LTC1966 IN1 VSS VOUT GND 1966 TA04 ±2.5V Supplies, Single Ended, DC-Coupled RMS-to-DC Converter with Shutdown 2.5V OFF ON –2V EN VDD LTC1966 IN1 VSS –2.5V VOUT GND –2.5V IN2 OUT RTN CAVE 1µF DC OUTPUT 0.1µF X7R –2.5V IN2 OUT RTN CAVE 1µF DC OUTPUT DC + AC INPUT (1VPEAK) 1966 TA06 Battery Powered Single-Ended AC-Coupled RMS-to-DC Converter AC INPUT (1VPEAK) 9V CC 0.1µF VDD LTC1966 IN1 VOUT IN2 OUT RTN VSS GND EN DC CAVE OUTPUT 1µF GND LT1175CS8-5 SHDN VIN OUT SENSE 0.1µF X7R 1966 TA07 1966fb 34 LTC1966 siMpliFieD scheMaTic VDD GND VSS C1 Y1 C2 IN1 2nd ORDER ∆∑ MODULATOR Y2 C12 IN2 C3 C5 C7 C9 OUTPUT + A1 C4 + A2 C8 C11 OUT RTN 1966 SS CAVE – C6 – C10 EN TO BIAS CONTROL CLOSED DURING SHUTDOWN 30k BLEED RESISTOR FOR CAVE 1966fb 35 LTC1966 package DescripTion (Reference LTC DWG # 05-08-1660 Rev F) MS8 Package 8-Lead Plastic MSOP 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 8 7 65 0.52 (.0205) REF 0.889 ± 0.127 (.035 ± .005) GAUGE PLANE 0.254 (.010) DETAIL “A” 0° – 6° TYP 4.90 ± 0.152 (.193 ± .006) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) DETAIL “A” 0.53 ± 0.152 (.021 ± .006) 0.18 (.007) SEATING PLANE 1 1.10 (.043) MAX 23 4 0.86 (.034) REF 0.42 ± 0.038 (.0165 ± .0015) TYP 0.65 (.0256) BSC RECOMMENDED SOLDER PAD LAYOUT NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 0.22 – 0.38 (.009 – .015) TYP 0.65 (.0256) BSC 0.1016 ± 0.0508 (.004 ± .002) MSOP (MS8) 0307 REV F 1966fb 36 LTC1966 revision hisTory REV B DATE 5/11 DESCRIPTION Revised entire data sheet to add H- and MP- grades (Revision history begins at Rev B) PAGE NUMBER 1 to 38 1966fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 37 LTC1966 Typical applicaTion RMS Noise Measurement 5V VOLTAGE NOISE IN 5V VDD 10k LTC1966 IN1 VOUT IN2 OUT RTN –5V 100 1.5µF 100k 0.1µF VSS GND EN –5V 1966 TA10 + 100 VOUT = – 1/2 LTC6203 1mVDC 1µVRMS NOISE CAVE 1µF BW 1kHz TO 100kHz INPUT SENSITIVITY = 1µVRMS TYP 70A Current Measurement 5V LTC1966 IN1 T1 10 VOUT IN2 OUT RTN VSS GND EN –5V 1966 TA09 Single Supply RMS Current Measurement V+ AC CURRENT 71.2A MAX 50Hz TO 400Hz CAVE 1µF VOUT 4mVDC/ARMS AC CURRENT 71.2A MAX 50Hz TO 400Hz LTC1966 IN1 T1 10 VOUT IN2 OUT RTN 100k VSS GND EN 0.1µF 100k 1966 TA08 CAVE 1µF VOUT = 4mVDC/ARMS T1: CR MAGNETICS CR8348-2500-N www.crmagnetics.com T1: CR MAGNETICS CR8348-2500-N www.crmagnetics.com relaTeD parTs PART NUMBER LT®1077 LT1175-5 LT1494 LT1782 LT1880 LTC1967 LTC1968 LTC2050 LT2178/LT2178A LTC2402 LTC2420 LTC2422 DESCRIPTION Micropower, Single Supply Precision Op Amp Negative, –5V Fixed, Micropower LDO Regulator 1.5µA Max, Precision Rail-to-Rail I/O Op Amp General Purpose SOT-23 Rail-to-Rail Op Amp SOT-23 Rail-to-Rail Output Precision Op Amp Precision, Extended Bandwidth RMS to DC Converter Precision, Wide Bandwidth RMS to DC Converter Zero Drift Op Amp in SOT-23 17µA Max, Single Supply Precision Dual Op Amp 2-Channel, 24-bit, Micropower, No Latency ∆S™ ADC 20-bit, Micropower, No Latency ∆S ADC in SO-8 2-Channel, 20-bit, Micropower, No Latency ∆S ADC COMMENTS 48µA ISY, 60µV VOS(MAX), 450pA IOS(MAX) 45µA IQ, Available in SO-8 or SOT-223 375µV VOS(MAX), 100pA IOS(MAX) 40µA ISY, 800µV VOS(MAX), 2nA IOS(MAX) 1.2mA ISY, 150µV VOS(MAX), 900pA IOS(MAX) 330µA ISY, ∆∑ RMS Conversion to 4MHz 2.3mA ISY, ∆∑ RMS Conversion to 15MHz 750µA ISY, 3µV VOS(MAX), 75pA IB(MAX) 14µA ISY, 120µV VOS(MAX), 350pA IOS(MAX) 200µA ISY, 4ppm INL, 10ppm TUE 200µA ISY, 8ppm INL, 16ppm TUE Dual Channel Version of LTC2420 1966fb 38 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0511 REV B • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORA TION 2001