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LTC1966

sn1966 1966fas

Precision Micropower,

∆Σ RMS-to-DC Converter

■

No-Hassle Simplicity:

True RMS-DC Conversion with Only One External

Capacitor

Delta Sigma Conversion 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 1V

PEAK

Differential Voltage

■

Flexible Output:

Rail-to-Rail Output

Separate Output Reference Pin Allows Level Shifting

■

Small Size:

Space Saving 8-Pin MSOP Package

■

True RMS Digital Multimeters and Panel Meters

■

True RMS AC + DC Measurements

DESCRIPTIO

FEATURES

APPLICATIO S

The LTC

1966 is a true RMS-to-DC converter that utilizes

an innovative patented ∆Σ computational technique. The

internal delta-sigma circuitry of the LTC1966 makes it sim-

pler 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. Differ-

ential input range is 1V

PEAK

, and offers unprecedented lin-

earity. 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.

Single Supply RMS-to-DC Converter

AVE

1µF

OUT

–

1966 TA01

2.7V TO 5.5V

OUTPUT

DIFFERENTIAL

INPUT

LTC1966

0.1µF

OPT. AC

COUPLING

EN GND

OUT RTN

IN1

IN2

TYPICAL APPLICATIO

(mV AC

RMS

)

–1.0

LINEARITY ERROR (V

OUT

mV DC – V

mV AC

RMS

)

–0.6

–0.2

0.2

–0.8

–0.4

100 200 300 400

1966 TA01b

500500 150 250 350 450

LTC1966, ∆Σ

60Hz SINEWAVES

CONVENTIONAL

LOG/ANTILOG

Quantum Leap in Linearity Performance

, LTC and LT are registered trademarks of Linear Technology Corporation.

Protected under U.S. Patent Numbers 6,359,576 and 6,362,677

Summary of content (32 pages)

PAGE 1
LTC1966 Precision Micropower, ∆Σ RMS-to-DC Converter DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ No-Hassle Simplicity: True RMS-DC Conversion with Only One External Capacitor Delta Sigma Conversion 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.
PAGE 2
LTC1966 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltage VDD to GND .............................................. – 0.3 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.
PAGE 3
LTC1966 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VDD = 5V, VSS = – 5V, VOUTRTN = 0V, CAVE = 10µF, VIN = 200mVRMS, VENABLE = 0.5V unless otherwise noted.
PAGE 4
LTC1966 ELECTRICAL CHARACTERISTICS 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.
PAGE 5
LTC1966 U W TYPICAL PERFOR A CE CHARACTERISTICS Gain and Offsets vs Temperature 0.3 VIOS 0.4 VDD = 5V 0.4 VSS = –5V 0.3 0.3 VOOS 0.1 0.1 GAIN ERROR 0 0 –0.1 –0.1 –0.2 –0.2 –0.3 –0.3 –0.4 –0.4 –0.5 –50 –25 50 75 25 0 TEMPERATURE (°C) 100 –0.5 125 0.4 0.3 0.2 0.2 GAIN ERROR 0.1 VOOS 0 0 VIOS –0.1 –0.1 –0.2 –0.2 –0.3 –0.3 –0.4 –0.4 –0.5 –50 –25 50 75 25 0 TEMPERATURE (°C) 100 1966 G08 1966 G09 1.0 VIOS VDD = 2.7V 0.4 VSS = GND 0.6 0.3 GAIN ERROR 0.1 0.
PAGE 6
LTC1966 U W TYPICAL PERFOR A CE CHARACTERISTICS Gain and Offset vs VDD Supply 0.8 0.6 0.4 0.2 0.1 GAIN ERROR 0 –0.1 0 – 0.2 VOOS –0.2 – 0.4 –0.3 – 0.6 –0.4 – 0.8 2.5 3.0 3.5 4.0 VDD (V) 4.5 5.0 200.6 20Hz 60Hz 200.4 100Hz 200.2 199.8 1.0 1.5 2.0 2.5 3.0 3.5 4.0 CREST FACTOR 4.5 0.20 VOUT (mV DC) – VIN (mV ACRMS) VOUTDC – |VINDC| (mV) –0.05 –0.10 0.02 0 –0.02 –0.04 EFFECT OF OFFSETS MAY BE POSITIVE OR NEGATIVE –300 100 –100 VIN1 (mV) 300 1966 G13 250 15 VDD = 2.
PAGE 7
LTC1966 U W TYPICAL PERFOR A CE CHARACTERISTICS 202 201 OUTPUT DC VOLTAGE (mV) 198 196 194 192 190 188 186 184 1%/DIV CAVE = 2.2µF 182 10 100 1 INPUT FREQUENCY (kHz) 30 0.
PAGE 8
LTC1966 U U U PI FU CTIO S 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: ( VOUT – OUT RTN) = OUT RTN (Pin 6): Output Return.
PAGE 9
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 10
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 11
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 12
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 13
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 14
LTC1966 U W U U APPLICATIO S I FOR ATIO 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 50 60 100 1966 F08 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.
PAGE 15
LTC1966 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 16
LTC1966 U W U U APPLICATIO S I FOR ATIO 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).
PAGE 17
LTC1966 U W U U APPLICATIO S I FOR ATIO 120 120 100 LTC1966 OUTPUT (mV) 100 LTC1966 OUTPUT (mV) CAVE = 1µF CAVE = 1µF 80 60 40 80 60 40 20 20 0 0 0 0.1 0.2 0.3 TIME (SEC) 0.4 0 0.5 0.2 0.4 0.6 TIME (SEC) 1 0.8 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.
PAGE 18
LTC1966 U W U U APPLICATIO S I FOR ATIO Reducing Ripple with a Post Filter The output ripple is always much larger than the DC error, so filtering out the ripple can reduce the peak error substantially, without the large settling time penalty of simply increasing the averaging capacitor. Figure 13 shows a basic 2nd order post filter, for a net 3rd order filtering of the LTC1966 RMS calculation.
PAGE 19
LTC1966 U W U U APPLICATIO S I FOR ATIO (≈0.7 • 1.8 ≈ 1.3Hz) than with 1µF alone. To adjust the bandwidth of either of them, simply scale all the capacitors by a common multiple, and leave the resistors unchanged. The step responses of the LTC1966 with 1µF-only and with the two post filters are shown in Figure 15. This is the rising edge RMS output response to a 10Hz input starting at t = 0.
PAGE 20
LTC1966 U W U U APPLICATIO S I FOR ATIO 0 –0.2 C = 10µF PEAK ERROR (%) –0.4 –0.6 C = 4.7µF 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) 1 100 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 C = 2.2µF C = 1.0µF C = 0.47µF C = 0.22µF C = 0.1µF –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 1 10 INPUT FREQUENCY (Hz) 100 1966 F18 Figure 18.
PAGE 21
LTC1966 U W U U APPLICATIO S I FOR ATIO 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) 10 100 1066 F20 Figure 20.
PAGE 22
LTC1966 U W U U APPLICATIO S I FOR ATIO Obviously, the effect of crest factor is somewhat simplified above given the factor of two difference based on a subjective description of the waveform type. The results will vary somewhat based on actual crest factor and waveform dynamics and the type of filtering used. The above method is conservative for some cases and about right for others. The LTC1966 works well with signals whose crest factor is 4 or less.
PAGE 23
LTC1966 U W U U APPLICATIO S I FOR ATIO VOUT = (√(50mV AC)2 + (0.2mV DC)2) • 1.001 + 0.1mV = 50.150mV = 50mV + 0.301% VOUT = (√(5mV AC)2 + (0.2mV DC)2) • 1.001 + 0.1mV = 5.109mV = 5mV + 2.18% As can be seen, the gain term dominates with large inputs, while the offset terms become significant with smaller inputs. In fact, 5mV is the minimum RMS level needed to keep the LTC1966 calculation core functioning normally, so this represents the worst-case of usable input levels.
PAGE 24
LTC1966 U W U U APPLICATIO S I FOR ATIO small, 2.5pF • 100kΩ = 250ns, and during the 2.5µs period devoted to sampling, ten time constants elapse. This allows each sample to settle to within 46ppm and it is these samples that are used to compute the RMS value. This is a much higher accuracy than the LTC1966 conversion limits, and far better than the accuracy computed via the simplistic resistive divider model: RIN VIN = VSOURCE RIN + RSOURCE 8MΩ = VSOURCE 8MΩ + 100kΩ = VSOURCE – 1.
PAGE 25
LTC1966 U W U U APPLICATIO S I FOR ATIO LTC1966 OUTPUT OUT RTN 7106 TYPE 5 31 6 CAVE 30 IN HI IN LO 1966 F22a Figure 22a. Interfacing to DVM/DPM ADC LTC1966 OUTPUT OUT RTN SYSTEM CALIBRATION LTC2420 5 6 3 CAVE 4 VIN SDO GND SCK CS 1966 F22b The DC-accurate filter of Figure 14 is attractive from an error standpoint, but it increases the impedance at the ADC input.
PAGE 26
LTC1966 U W U U APPLICATIO S I FOR ATIO The best frequency for the calibration signal is roughly ten times the – 0.1% DC error frequency. For 1µF, – 0.1% DC error occurs at 8Hz, so 80Hz is a good calibration frequency, although anywhere from 60Hz to 100Hz should suffice. The trade-off here is that on the one hand, the DC error is input frequency dependent, so a calibration signal frequency high enough to make the DC error negligible should be used.
PAGE 27
LTC1966 U W U U APPLICATIO S I FOR ATIO 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. GROUND PIN 6 2.
PAGE 28
LTC1966 U W U U APPLICATIO S I FOR ATIO 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. 8. Large errors occur at crest factors approaching, but less than 4. – Insufficient averaging. Solution: Increase CAVE.
PAGE 29
LTC1966 U TYPICAL APPLICATIO S ±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 IN2 OUT RTN CAVE 1µF AC INPUTS (1VPEAK DIFFERENTIAL) DC OUTPUT VSS GND EN –5V IN1 VOUT IN2 OUT RTN CC 0.1µF CAVE 1µF DC OUTPUT VSS GND EN 1966 TA05 1966 TA03 2.7V Single Supply, Single Ended, AC-Coupled RMS-to-DC Converter with Shutdown 2.
PAGE 30
LTC1966 W W SI PLIFIED SCHE ATIC VDD C12 GND VSS C1 ∫ Y1 ∫ Y2 C2 IN1 2nd ORDER ∆Σ MODULATOR IN2 C3 C5 C7 + C9 + A1 C4 OUTPUT – A2 C8 CAVE C11 – OUT RTN 1966 SS C6 C10 CLOSED DURING SHUTDOWN EN TO BIAS CONTROL 30k BLEED RESISTOR FOR CAVE sn1966 1966fas 30
PAGE 31
LTC1966 U PACKAGE DESCRIPTIO MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.2 – 3.45 (.126 – .136) 0.42 ± 0.04 (.0165 ± .0015) TYP 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.65 (.0256) BSC 8 7 6 5 0.52 (.206) REF RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 3.00 ± 0.102 (.118 ± .004) NOTE 4 4.90 ± 0.15 (1.93 ± .006) DETAIL “A” 0° – 6° TYP GAUGE PLANE 0.53 ± 0.015 (.021 ± .006) DETAIL “A” 1 2 3 4 1.10 (.043) MAX 0.86 (.034) REF 0.
PAGE 32
LTC1966 U TYPICAL APPLICATIO S ±2.5V Supplies, Single Ended, DC-Coupled RMS-to-DC Converter with Shutdown 0.1µF X7R 2.5V ≥2V OFF ON 5V –2.5V ≤–2V DC + AC INPUT (1VPEAK) RMS Noise Measurement VOLTAGE NOISE IN 5V EN VDD LTC1966 IN1 VDD + VOUT CAVE 1µF IN2 OUT RTN VSS GND –2.5V –2.5V DC OUTPUT 100Ω 10k 1/2 LTC6203 IN1 – VOUT IN2 OUT RTN 100Ω CAVE 1µF 0.1µF 1966 TA10 –5V 100k BW ≈ 1kHz TO 100kHz INPUT SENSITIVITY = 1µVRMS TYP 1.