LTC1966 Precision Micropower ∆∑ RMS-to-DC Converter Features n n n n n n n n n n n Simple to Use, Requires One Capacitor True RMS DC Conversion Using ∆Σ 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.
LTC1966 Absolute Maximum Ratings Pin Configuration (Note 1) 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.........
LTC1966 Electrical Characteristics 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 CONDITIONS PSRR (Note 9) LTC1966C, LTC1966I LTC1966H, LTC1966MP VIOS Power Supply Rejection Input Offset Voltage MIN TYP MAX UNITS 0.02 0.15 0.20 0.3 %V %V %V 0.02 0.8 1.
LTC1966 Electrical Characteristics 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 CONDITIONS IIL ENABLE Pin Current Low VENABLE = 0.5V LTC1966H, LTC1966MP VTH ENABLE Threshold Voltage VDD = 5V, VSS = –5V VDD = 5V, VSS = GND VDD = 2.
LTC1966 Typical Performance Characteristics VDD = 5V VSS = –5V 0.5 0.5 0.4 0.4 0.2 0.1 0 –0.1 –0.2 GAIN ERROR 0.1 0 VOOS –0.1 VIOS –0.2 VIOS 0.3 0.2 0.4 0.3 0.2 VOOS 0.1 0.1 GAIN ERROR 0 0 –0.1 –0.1 –0.2 –0.2 1.0 VDD = 2.7V VSS = GND 0.8 VIOS 0.3 0.2 0.6 0.4 GAIN ERROR 0.1 0.2 0 0 –0.1 –0.2 VOOS –0.2 –0.4 –0.3 –0.3 –0.3 –0.3 –0.6 –0.4 –0.4 –0.4 –0.4 –0.4 –0.8 –0.5 –0.5 –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) –0.
LTC1966 Typical Performance Characteristics VDD = 5V 0.1 VIOS VOOS GAIN ERROR 0.3 0.3 0.2 0.1 0 0 – 0.1 NOMINAL SPECIFIED CONDITIONS –0.1 –0.2 –0.3 –0.4 –0.5 0.4 –6 –5 – 0.2 –2 –3 VSS (V) –4 0 –1 VSS = GND 0.6 VIOS 0.2 0.4 0.2 0.1 GAIN ERROR 0 –0.1 0 – 0.2 VOOS –0.2 – 0.4 – 0.3 –0.3 – 0.6 – 0.4 –0.4 – 0.8 –0.5 –0.5 2.5 3.0 3.5 4.0 VDD (V) 4.5 5.0 200 250Hz 100Hz 190 180 200mVRMS SCR WAVEFORMS = 4.7µF C 160 VAVE= 5V DD 5%/DIV 150 6 2 3 5 4 1 10 0.15 0.
LTC1966 Typical Performance Characteristics Quiescent Supply Currents vs Temperature Input Signal Bandwidth 150 VDD = 5V, VSS = GND 30 VDD = 2.7V, VSS = GND 25 20 130 120 15 VDD = 5V, VSS = –5V 110 100 VDD = 5V, VSS = GND 0.1% ERROR 35 VDD = 2.7V, VSS = GND 10 1% ERROR 10% ERROR –3dB 10 1 100 10K 100K 1K INPUT SIGNAL FREQUENCY (Hz) 194 192 190 188 186 1M 30 0.
LTC1966 Pin Functions GND (Pin 1): Ground. A power return pin. VSS (Pin 4): Negative Voltage Supply. GND to – 5.5V. 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. VOUT (Pin 5): Output Voltage.
LTC1966 Applications Information START NOT SURE READ RMS-TO-DC CONVERSION DO YOU NEED TRUE RMS-TO-DC CONVERSION? FIND SOMEONE WHO DOES AND GIVE THEM THIS DATA SHEET NO YES CONTACT LTC BY PHONE OR AT www.linear.com AND GET SOME NOW DO YOU HAVE ANY LTC1966s YET? NO YES DID YOU ALREADY TRY OUT THE LTC1966? DO YOU WANT TO KNOW HOW TO USE THE LTC1966 FIRST? NO YES READ THE TROUBLESHOOTING GUIDE.
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.
LTC1966 Applications Information How an RMS-to-DC Converter Works How the LTC1966 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.
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 ∆Σ 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.
LTC1966 Applications Information 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.
LTC1966 Applications Information 0 –0.2 PEAK ERROR (%) –0.4 C = 100µF –0.6 –0.8 C = 47µF –1.0 C = 22µF C = 10µF C = 2.2µF C = 4.7µF C = 1µF –1.2 –1.4 –1.6 –1.8 –2.0 1 10 INPUT FREQUENCY (Hz) 20 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 <1% and the DC error will be <0.1% with frequencies of 10Hz or more.
LTC1966 Applications Information For single-ended DC-coupled applications, simply connect one of the two inputs (they are interchangeable) to the signal, and the other to ground. This will work well for dual supply configurations, but for single supply configurations it will only work well for unipolar input signals.
LTC1966 Applications Information 0 In any configuration, the averaging capacitor should be connected between Pins 5 and 6. The LTC1966 RMS DC output will be a positive voltage created at VOUT (Pin 5) with respect to OUT RTN (Pin 6). The LTC1966 is a switched capacitor device, and large transient power supply currents will be drawn as the switching occurs. For reliable operation, standard power supply bypassing must be included. For single supply operation, a 0.
LTC1966 Applications Information 120 120 CAVE = 1µF 100 LTC1966 OUTPUT (mV) LTC1966 OUTPUT (mV) 100 80 60 40 80 60 40 20 20 0 CAVE = 1µF 0 0.1 0.2 0.3 TIME (SEC) 0.4 0 0.5 0 0.2 0.4 0.6 TIME (SEC) 0.8 1 1966 F11b 1966 F11a Figure 11a. LTC1966 Rising Edge with CAVE = 1µF Figure 11b. LTC1966 Falling Edge with CAVE = 1µF SETTLING ACCURACY (%) 10 C = 0.1µF C = 0.22µF C = 0.47µF C = 1µF C = 2.2µF C = 4.7µF C = 10µF C = 22µF C = 47µF C = 100µF 1 0.1 0.01 0.
LTC1966 Applications Information But with 100µF, the settling time to even 10% is a full 38 seconds, which is a long time to wait. What can be done about such a design? If the reason for choosing 100µF is to keep the DC error with a 75mHz input less than 0.1%, the answer is: not much. The settling time to 1% of 76 seconds is just 5.7 cycles of this extremely low frequency. Averaging very low frequency signals takes a long time.
LTC1966 Applications Information Step Responses with a Post Filter Both of the post filters, shown in Figures 13 and 14, are optimized for additional filtering with clean step responses. The 85kΩ output impedance of the LTC1966 working into a 1µF capacitor forms a 1st order LPF with a –3dB frequency of ~1.8Hz. The two filters have 1µF at the LTC1966 output for easy comparison with a 1µF only case, and both have the same relative (Bessel-like) shape.
LTC1966 Applications Information 0 –0.2 C = 10µF PEAK ERROR (%) –0.4 –0.6 C = 4.7µF –0.8 C = 2.2µF C = 1.0µF C = 0.47µF C = 0.22µF C = 0.1µF –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 10 INPUT FREQUENCY (Hz) 1 100 1966 F17 Figure 17. Peak Error vs Input Frequency with Buffered Post Filter 0 C = 10µF –0.2 PEAK ERROR (%) –0.4 C = 4.7µF –0.6 C = 2.2µF C = 1.0µF C = 0.47µF C = 0.22µF C = 0.1µF –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 10 INPUT FREQUENCY (Hz) 1 Figure 18.
LTC1966 Applications Information SETTLING ACCURACY (%) 10 C = 0.1µF C = 0.22µF C = 0.47µF C = 1.0µF C = 2.2µF C = 4.7µF C = 10µF C = 22µF C = 47µF C = 100µF 1 0.1 0.01 0.1 1 SETTLING TIME (SEC) 10 100 1066 F14 Figure 19. Settling Time with Buffered Post Filter SETTLING ACCURACY (%) 10 C = 0.1µF C = 0.22µF C = 0.47µF C = 1.0µF C = 2.2µF C = 4.7µF C = 10µF C = 22µF C = 47µF C = 100µF 1 0.1 0.01 0.1 1 SETTLING TIME (SEC) 10 Figure 20.
LTC1966 Applications Information using the same design curves presented in Figures 6, 8, 17 and 18. For the worst-case of square top pulse trains, that are always either zero volts or the peak voltage, base the selection on the lowest fundamental input frequency divided by twice as much: fINPUT(MIN) fDESIGN = 6 • CF – 2 The effects of crest factor and DC offsets are cumulative. So for example, a 10% duty cycle pulse train from 0VPEAK to 1VPEAK (CF = √10 = 3.16) repeating at 16.
LTC1966 Applications Information But with 10× less AC input, the error caused by VIOS is 100× larger: VOUT = √(20mV AC)2 + (0.2mV DC)2 = 20.001mV = 20mV + 50ppm This phenomena, although small, is one source of the LTC1966’s residual nonlinearity. On the other hand, if the input is DC-coupled, the input offset voltage adds directly. With +200mV and a +0.2mV VIOS, a 200.2mV output will result, an error of 0.1% or 1000ppm.
LTC1966 Applications Information LTC1966 IN2 10 5mV MIN ASYMPTOTES SHIFTED +2.5mV 5 OUTPUT 170kΩ RMS TO DC CONVERSION 15 IINJECT DC CHARGE PUMP IN1 20 VOUT (mV DC) To do this, inject current into the output. As shown in Figure 21, the charge pump output impedance is 170kΩ, with the computational feedback cutting the closed loop output impedance to the 85kΩ specification. By injecting 30nA of current into this 170Ω, with zero input, a 5mV offset IDEAL 0 CAVE 5 0 Figure 21.
LTC1966 Applications Information The two 10MΩ resistors not connected to the supply can be any value as long as they match and the feed voltage is changed for 30nA injection. The op amp gain is only 1.00845, so the output is dominated by the LTC1966 RMS results, which keeps errors low. With the values shown, the resistors can be ±2% and only introduce ±170ppm of gain error. The 84.
LTC1966 Applications Information Input Impedance The LTC1966 true RMS-to-DC converter utilizes a 2.5pF capacitor to sample the input at a nominal 100kHz sample frequency. This accounts for the 8MΩ input impedance. See Figure 24 for the equivalent analog input circuit. Note however, that the 8MΩ input impedance does not directly affect the input sampling accuracy.
LTC1966 Applications Information However, resistive loading is an issue and the 10MΩ impedance of a DMM or 10× scope probe will drag the output down by –0.85% typ. During shutdown, the switching action is halted and a fixed 30k resistor shunts VOUT to OUT RTN so that CAVE is discharged. Guard Ringing the Output The LTC1966’s combination of precision and high output impedance can present challenges that make the use of a guard ring around the output a good idea for many applications.
LTC1966 Applications Information Interfacing with an ADC The LTC1966 output impedance and the RMS averaging ripple need to be considered when using an analog-todigital converter (ADC) to digitize the LTC1966 RMS result. The simplest configuration is to connect the LTC1966 directly to the input of a type 7106/7136 ADC as shown in Figure 25a. These devices are designed specifically for DVM/DPM use and include display drivers for a 3 1/2 digit LCD segmented display.
LTC1966 Applications Information As is shown in Figure 25b, where the LTC2420 is set to continuously convert by grounding the CS pin. The gain error will be less if CS is driven at a slower rate, however, the rate should either be consistent or at a rate low enough that the LTC1966 and its output capacitor have fully settled by the beginning of each conversion, so that the loading errors are consistent. Note that in this circuit, the input current of the LTC2420 is being used to assure monotonicity.
LTC1966 Applications Information Whatever calibration scheme is used, the linearity of the LTC1966 will improve the calibrated accuracy over that achievable with older log/antilog RMS-to-DC converters. Additionally, calibration using DC reference voltages are essentially as accurate with the LTC1966 as those using AC reference voltages. Older log/antilog RMS-to-DC converters required nonlinear input stages (rectifiers) whose linearity would typically render DC based calibration unworkable.
LTC1966 Applications Information The calculations of the error terms for a 200mV full-scale case are: Gain = Reading at 200mV – Reading at 20mV 180mV Output Offset = Reading at 20mV – 20mV Gain DC, 2 Point DC based calibration is preferable in many cases because a DC voltage of known, good accuracy is easier to generate than such an AC calibration voltage. The only down side is that the LTC1966 input offset voltage plays a role.
LTC1966 Applications Information Troubleshooting Guide Top Ten LTC1966 Application Mistakes 1. Circuit won’t work–Dead On Arrival–no power drawn. – Probably forgot to enable the LTC1966 by pulling Pin 8 low. 4. Gain is low by a few percent, along with other screwy results. – Probably tried to use output in a floating, differential manner. Solution: Tie Pin 6 to a low impedance. See Output Connections in the Design Cookbook. Solution: Tie Pin 8 to Pin 1. 2. Circuit won’t work, but draws power.
LTC1966 Applications Information 7. Output is noisy with >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 ∆Σ noise. Solution: Bandwidth limit the input or digitally filter the resulting output. 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.
LTC1966 Typical Applications ±5V Supplies, Differential, DC-Coupled RMS-to-DC Converter 5V Single Supply, Differential, AC-Coupled RMS-to-DC Converter 5V 5V DC + AC INPUTS (1VPEAK DIFFERENTIAL) VDD VDD LTC1966 LTC1966 IN1 VOUT CAVE 1µF IN2 OUT RTN AC INPUTS (1VPEAK DIFFERENTIAL) DC OUTPUT VSS GND EN IN1 VOUT IN2 OUT RTN CC 0.1µF 1966 TA05 ±2.5V Supplies, Single Ended, DC-Coupled RMS-to-DC Converter with Shutdown 2.
LTC1966 Simplified Schematic VDD C12 GND VSS C1 Y1 Y2 C2 IN1 2nd ORDER ∆∑ MODULATOR IN2 C3 C5 C7 + C9 + A1 C4 OUTPUT – A2 C8 CAVE C11 – OUT RTN 1966 SS C6 EN TO BIAS CONTROL C10 CLOSED DURING SHUTDOWN 30k BLEED RESISTOR FOR CAVE 1966fb 35
LTC1966 Package Description MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660 Rev F) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 0.254 (.010) 7 6 5 0.52 (.0205) REF 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0° – 6° TYP GAUGE PLANE 3.20 – 3.45 (.126 – .136) 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 0.42 ± 0.038 (.0165 ± .0015) TYP 8 0.65 (.0256) BSC 1 1.10 (.043) MAX 2 3 4 0.86 (.034) REF 0.18 (.
LTC1966 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION B 5/11 Revised entire data sheet to add H- and MP- grades 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.
LTC1966 Typical Application RMS Noise Measurement 5V VOLTAGE NOISE IN 5V VDD + 100Ω 10k 1/2 LTC6203 IN1 – 1mVDC 1µVRMS NOISE VOUT CAVE 1µF IN2 OUT RTN –5V 100Ω VSS GND EN 0.1µF –5V 100k 1966 TA10 BW 1kHz TO 100kHz INPUT SENSITIVITY = 1µVRMS TYP 1.5µF AC CURRENT 71.2A MAX 50Hz TO 400Hz VOUT = LTC1966 70A Current Measurement Single Supply RMS Current Measurement 5V V+ LTC1966 IN1 T1 VOUT 10Ω CAVE 1µF IN2 OUT RTN VOUT 4mVDC/ARMS AC CURRENT 71.
