LTC1967IMS8

FEATURES s s LTC1967 Precision Extended Bandwidth, RMS-to-DC Converter DESCRIPTIO The LTC®1967 is a true RMS-to-DC converter that uses an innovative delta-sigma computational technique. The benefits of the LTC1967 proprietary architecture when compared to conventional log-antilog RMS-to-DC converters are higher linearity and accuracy, bandwidth independent of amplitude and improved temperature behavior. The LTC1967 operates with 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. The LTC1967 allows hassle-free system calibration at any input voltage. The LTC1967 has a rail-to-rail output with a separate output reference pin providing flexible level shifting; it operates on a single power supply from 4.5V to 5.5V. A low power shutdown mode reduces supply current to 0.1µA. The LTC1967 is packaged in the space-saving MSOP package, which is ideal for portable applications. , LTC and LT are registered trademarks of Linear Technology Corporation. Protected under U.S. Patent Numbers 6,359,576, 6,362,677 and 6,516,291 s s s s s s High Linearity: 0.02% Linearity Allows Simple System Calibration Wide Input Bandwidth: Bandwidth to 0.1% Additional Gain Error: 40kHz Bandwidth Independent of Input Voltage Amplitude No-Hassle Simplicity: True RMS-DC Conversion with Only One External Capacitor Delta Sigma Conversion Technology Low Supply Current: 330µA Typ Ultralow Shutdown Current: 0.1µA 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 Small Size: Space Saving 8-Pin MSOP Package APPLICATIO S s s True RMS Digital Multimeters and Panel Meters True RMS AC + DC Measurements TYPICAL APPLICATIO Linearity Performance LINEARITY ERROR (VOUT mV DC – VIN mV ACRMS) 0.2 LTC1967, ∆Σ 0 –0.2 –0.4 –0.6 –0.8 –1.0 60Hz SINEWAVE 0 100 200 300 VIN (mV ACRMS) 400 500 1967 TA01b Single Supply RMS-to-DC Converter 4.5V TO 5.5V V+ IN1 DIFFERENTIAL INPUT 0.1µF OPT. AC COUPLING IN2 EN OUTPUT LTC1967 OUT RTN GND CAVE 1µF + VOUT – 1967 TA01 U U U CONVENTIONAL LOG/ANTILOG 1967f 1 LTC1967 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW GND IN1 IN2 NC 1 2 3 4 8 7 6 5 ENABLE V+ OUT RTN VOUT Supply Voltage V+ to GND ............................................................. 6V Input Currents (Note 2) ..................................... ±10mA Output Current (Note 3) ..................................... ± 10mA ENABLE Voltage ......................................... –0.3V to 6V OUT RTN Voltage ........................................ –0.3V to V+ Operating Temperature Range (Note 4) LTC1967C/LTC1967I ......................... – 40°C to 85°C Specified Temperature Range (Note 5) LTC1967C/LTC1967I ......................... – 40°C to 85°C Maximum Junction Temperature ......................... 150°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C LTC1967CMS8 LTC1967IMS8 MS8 PART MARKING LTTJ MS8 PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 220°C/ W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS SYMBOL GERR VOOS ∆VOOS/∆T LINERR PSRRG VIOS ∆VIOS/∆T PARAMETER Low Frequency Gain Error Output Offset Voltage Output Offset Drift Linearity Error Power Supply Rejection Input Offset Voltage Input Offset Drift CF = 3 CF = 5 Input Characteristics VIMAX IVR ZIN CMRRI VIMIN PSRRI Maximum Peak Input Swing Input Voltage Range Input Impedance Input Common Mode Rejection Minimum RMS Input Power Supply Rejection Conversion Accuracy The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. V+ = 5V, VOUTRTN = 2.5V, CAVE = 10µF, VIN = 200mVRMS, VENABLE = 0.5V unless otherwise noted. CONDITIONS 50Hz to 5kHz Input (Notes 6, 7) q MIN TYP ± 0.1 0.1 MAX ± 0.3 ± 0.4 0.55 10 0.15 0.15 0.20 1.5 10 UNITS % % mV µV/°C % %/V %/V mV µV/°C mV mV V (Notes 6, 7) (Note 11) 50mV to 350mV (Notes 7, 8) (Note 9) q q q 2 0.02 0.02 0.2 (Notes 6, 7, 10) (Note 11) 60Hz Fundamental, 200mVRMS 60Hz Fundamental, 200mVRMS Accuracy = 1% (Note 14) Average, Differential (Note 12) Average, Common Mode (Note 12) (Note 13) (Note 9) q q q q 1 0.2 5 1 0 5 100 50 250 1.05 Additional Error vs Crest Factor (CF) q q q q V+ 400 5 600 2 U V MΩ MΩ µV/V mV µV/V 1967f W U U WW W LTC1967 ELECTRICAL CHARACTERISTICS SYMBOL OVR ZOUT CMRRO VOMAX PSRRO f1P f– 3dB V+ IS PARAMETER Output Voltage Range Output Impedance Output Common Mode Rejection Maximum Differential Output Swing Power Supply Rejection 0.1% Additional Gain Error (Note 15) ± 3dB Frequency (Note 15) Supply Voltage Supply Current IN1 = 20mV, IN2 = 0V IN1 = 200mV, IN2 = 0V VENABLE = 4.5V VENABLE = 4.5V VENABLE = 0.5V q q The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. V+ = 5V, VOUTRTN = 2.5V, CAVE = 10µF, VIN = 200mVRMS, VENABLE = 0.5V unless otherwise noted. CONDITIONS q MIN 0 40 1.0 0.9 TYP MAX V+ UNITS V kΩ µV/V V V Output Characteristics (Note 12) (Note 13) Accuracy = 1%, DC Input (Note 14) q q q 50 50 1.05 250 40 4 65 250 (Note 9) q 1000 µV/V kHz MHz Frequency Response Power Supplies 4.5 320 340 0.1 –1 –3 – 0.1 –0.5 2.1 0.1 – 0.1 5.5 390 V µA µA µA µA µA V V Shutdown Characteristics ISS IIH IIL VTH VHYS Supply Current ENABLE Pin Current High ENABLE Pin Current Low ENABLE Threshold Voltage ENABLE Threshold Hysteresis q q q 10 Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The inputs (IN1, IN2) are protected by shunt diodes to GND and V+. If the inputs are driven beyond the rails, the current should be limited to less than 10mA. Note 3: The LTC1967 output (VOUT) is high impedance and can be overdriven, either sinking or sourcing current, to the limits stated. Note 4: The LTC1967C/LTC1967I are guaranteed functional over the operating temperature range of – 40°C to 85°C. Note 5: The LTC1967C is guaranteed to meet specified performance from 0°C to 70°C. The LTC1967C 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 LTC1967I is guaranteed to meet specified performance from – 40°C to 85°C. Note 6: High speed automatic testing cannot be performed with CAVE = 10µF. The LTC1967 is 100% tested with CAVE = 47nF. Correlation tests have shown that the performance limits 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 LTC1967 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 can be guaranteed with the additional testing being performed to guarantee proper operation of all internal circuitry. Note 8: The LTC1967 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 LTC1967 are measured with DC inputs from 50mV to 350mV. The change in accuracy from V+ = 4.5V to V+ = 5.5V is divided by 1V. 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 LTC1967 behavior is simpler to characterize and the input offset voltage is the only significant source of “DC reversal error.” Note 11: Guaranteed by design. Note 12: The LTC1967 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. 1967f 3 LTC1967 ELECTRICAL CHARACTERISTICS Note 13: The common mode rejection ratios of the LTC1967 are measured with DC inputs from 50mV to 350mV. The input CMRR is defined as the change in VIOS measured between input levels of 0V to 350mV and input levels of V+ – 350mV to V+ divided by V+ – 350mV. The output CMRR is defined as the change in VOOS measured with OUT RTN = 0V and OUT RTN = V+ – 350mV divided by V+ – 350mV. Note 14: The LTC1967 input and output voltage swings are limited by internal clipping. However, its ∆Σ topology is relatively tolerant of momentary internal clipping. Note 15: The LTC1967 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 LTC1967 is inherently wideband, but the output accuracy is degraded by this aliased noise. TYPICAL PERFOR A CE CHARACTERISTICS Gain and Offset vs Input Common Mode Voltage 0.3 0.2 0.1 GAIN ERROR (%) 50mV ≤ VIN(PEAK) ≤ 350mV GAIN ERROR VIOS GAIN ERROR (%) 0 –0.1 –0.2 –0.3 –0.4 –0.5 –0.6 –0.7 –1.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT COMMON MODE VOLTAGE (V) 1967 G01 Gain and Offsets vs Temperature 0.05 0.04 0.03 50mV ≤ VIN(PEAK) ≤ 350mV 0.5 0.4 0.3 GAIN ERROR (%) GAIN ERROR (%) 0.02 0.01 0 –0.01 –0.02 –0.03 –0.04 –0.05 –40 –15 GAIN ERROR 35 10 TEMPERATURE (°C) 4 UW Gain and Offset vs Output Common Mode Voltage 1.0 0.8 0.6 OFFSET VOLTAGE (mV) 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 VIOS VOOS 50mV ≤ VIN(PEAK) ≤ 350mV GAIN ERROR 1.0 0.8 0.6 OFFSET VOLTAGE (mV) 0.4 0.2 0 VOOS –0.2 –0.4 –0.6 –0.8 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 OUTPUT COMMON MODE VOLTAGE (V) 1967 G02 Gain and Offset vs Supply Voltage 0.5 0.4 0.3 OFFSET VOLTAGE (mV) 50mV ≤ VIN(PEAK) ≤ 350mV 1.0 0.8 0.6 OFFSET VOLTAGE (mV) VOOS 0.2 0.1 0 –0.1 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 4.5 4.8 GAIN ERROR VOOS VIOS 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 VIOS –0.2 –0.3 –0.4 60 85 1967 G03 –0.5 5.7 5.4 5.1 SUPPLY VOLTAGE (V) –1.0 6.0 1967 G04 1967f LTC1967 TYPICAL PERFOR A CE CHARACTERISTICS Performance vs Crest Factor 200mVRMS SCR WAVEFORMS 200.8 CAVE = 10µF O.1%/DIV 200.6 200.4 200.2 200.0 199.8 199.6 199.4 199.2 199.0 1 2 3 CREST FACTOR 20Hz 60Hz 1kHz 201.0 220 210 OUTPUT VOLTAGE (mV DC) OUTPUT VOLTAGE (mV DC) 200 190 180 170 160 150 140 200mVRMS SCR WAVEFORMS 130 CAVE = 10µF 5%/DIV 120 6 2 3 5 4 1 CREST FACTOR 60Hz 10kHz VOUT (mV DC) – VIN (mV ACRMS) DC Linearity CAVE = 1µF 0.08 VIN2 = MIDSUPPLY 0.06 {VOUTDC – |VINDC|} (mV) 0.10 450 400 SUPPLY CURRENT (µA) 0.04 0.02 0 –0.02 –0.04 –0.06 EFFECTS OF OFFSETS –0.08 MAY BE POSITIVE OR NEGATIVE AT VIN = 0V –0.10 –300 100 –500 –100 VIN1 (mV) SUPPLY CURRENT (µA) Shutdown Current vs ENABLE Voltage 500 400 SUPPLY CURRENT (µA) IS 300 IEN 200 100 0 –100 OUTPUT DC VOLTAGE (mV) OUTPUT DC VOLTAGE (mV) 100 0 –100 0 1 4 3 5 2 ENABLE PIN VOLTAGE (V) 6 1967 G11 UW 4 300 1967 G08 Performance vs Large Crest Factors 0.20 AC Linearity 0.15 0.10 0.05 0 60Hz SINEWAVES CAVE = 10µF VIN2 = MIDSUPPLY 10Hz 20Hz 1kHz –0.05 –0.10 –0.15 –0.20 0 100 200 300 VIN1 (mV ACRMS) 400 500 1967 G07 5 1967 G05 7 8 1967 G06 Supply Current vs Supply Voltage 345 340 350 300 250 200 150 100 50 0 500 0 1 2 4 3 SUPPLY VOLTAGE (V) 5 6 1967 G09 Supply Current vs Temperature VS = 5V 335 330 325 320 315 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1967 G10 Input Signal Bandwidth vs RMS Value 300 200 ENABLE PIN CURRENT (nA) Input Signal Bandwidth 10% ERROR 1000 0.1% ERROR 1% ERROR 202 200 198 196 194 192 190 188 186 184 1%/DIV CAVE = 1µF 182 1k 10k 100k 1M 100 INPUT SIGNAL FREQUENCY (Hz) 100 –200 –300 –400 10 –3dB 1 100 1k 10k 100k 1M INPUT SIGNAL FREQUENCY (Hz) 10M 1967 G12 10M 1967 G13 1967f 5 LTC1967 TYPICAL PERFOR A CE CHARACTERISTICS Bandwidth to 200kHz 202 201 0.5%/DIV CAVE = 47µF OUTPUT VOLTAGE (mV) INPUT CMRR (dB) 200 VOUT (mV DC) 199 198 197 196 195 0 50k 150k INPUT FREQUENCY (Hz) 100k 200k 1967 G14 Output Accuracy vs Signal Amplitude 10 1% ERROR VIN2 = MIDSUPPLY PEAK OUTPUT NOISE (% OF READING) 1 {VOUT (mV DC) – VIN (mVRMS)} (mV) 5 0 –5 –1% ERROR –10 –15 –20 0 0.5 1 VIN1 (VRMS) 1.5 2 1967 G17 6 UW DC Transfer Function Near Zero 40 35 30 25 20 15 10 5 0 –5 –10 –30 –20 0 10 –10 VIN1 (mV DC) 20 30 1967 G15 Input Common Mode Rejection Ratio vs Frequency 100 90 80 70 60 50 40 30 20 10 0 10 100 10k 100k 1M 1k INPUT FREQUENCY (Hz) 10M 1967 G16 VIN2 = MIDSUPPLY THREE REPRESENTATIVE UNITS 4.5V COMMON MODE INPUT CONVERSION TO DC OUTPUT Output Noise vs Input Frequency PEAK NOISE DURING 10 SECOND MEASUREMENT 0.1 CAVE = 1µF CAVE = 10µF DC 0.01 CAVE = 100µF AC – 60Hz SINEWAVE 0.001 1k 10k INPUT FREQUENCY (Hz) 100k 1967 G18 1967f LTC1967 PI FU CTIO S GND (Pin 1): Ground. The power return pin. IN1 (Pin 2): Differential Input. DC coupled (polarity is irrelevant). IN2 (Pin 3): Differential Input. DC coupled (polarity is irrelevant). 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 often tied to GND, it can be tied to any arbitrary voltage: GND < OUT RTN < (V+ – Max Output) V+ (Pin 7): Positive Voltage Supply. 4.5V to 5.5V. ENABLE (Pin 8): An Active-Low Enable Input. LTC1967 is debiased if open circuited or driven to V+. For normal operation, pull to GND. ( VOUT – OUT RTN) = 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. Mathematically, RMS is the “Root of the Mean of the Square”: VRMS = V2 1V DC + – R 1V ACRMS R SAME HEAT 1V (AC + DC) RMS R 1967 F01 Figure 1 U W UU U U U 2 Average (IN2 – IN1)      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 AVERAGE RECTIFIED (V) 1.000 0.900 0.866 0.637 0.536 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 ERROR* 11% *Calibrate for 0% Error –3.8% –29.3% –40.4% 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 1967f 7 LTC1967 APPLICATIO S I FOR ATIO 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 LTC1967 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 non-sinusoidal 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 + – VTHY + VLINE ILOAD CONTROL – 1967 F02a Figure 2a VLINE Θ VLOAD VTHY ILOAD 1967 F02b Figure 2b 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 8 U 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 =  ,  VOUT    Because VOUT is DC, 2  ( V )2   ( VIN )    IN , so =   VOUT  VOUT   W UU VOUT  ( V )2   IN  = , and VOUT ( VOUT )2 = ( VIN )2, or VOUT = ( VIN )2 = RMS( VIN ) (VIN )2 VOUT VIN ×÷ LPF VOUT 1967 F03 Figure 3. RMS-to-DC Converter with Implicit Computation Unlike the prior generation RMS-to-DC converters, the LTC1967 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. How the LTC1967 RMS-to-DC Converter Works The LTC1967 uses a completely new topology for RMS-toDC conversion, in which a ∆Σ modulator acts as the divider, and a simple polarity switch is used as the multiplier1 as shown in Figure 4. 1Protected by multiple patents. 1967f LTC1967 APPLICATIO S I FOR ATIO Dα VIN VOUT ∆-Σ REF VIN ±1 LPF 1967 F04 Figure 4. Topology of LTC1967 The ∆Σ 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 ∆Σ 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 equal 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 LTC1967 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. More detail of the LTC1967 inner workings is shown in the Simplified Schematic towards the end of this data sheet. INPUT CIRCUITRY • VIOS • INPUT NONLINEARITY INPUT Figure 5. Linearity Model of an RMS-to-DC Converter 1967f U 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 VOUT W UU 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-toDC 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. IDEAL RMS-TO-DC CONVERTER OUTPUT CIRCUITRY • VOOS • OUTPUT NONLINEARITY OUTPUT 1967 F05 9 LTC1967 APPLICATIO S I FOR ATIO 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 LTC1967 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 LTC1967. DESIGN COOKBOOK The LTC1967 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 LTC1967 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 LTC1967 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 0 –0.2 –0.4 –0.6 DC ERROR (%) –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 1 C = 2.2µF C = 1µF C = 10µF C = 4.7µF C = 22µF OUTPUT Figure 6. DC Error vs Input Frequency 1967f 10 U 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 values2. 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 ∆Σ A/D converter or even a mechanical analog meter. 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 2This frequency-dependent error is in additon 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. W UU ACTUAL OUTPUT WITH RIPPLE f = 2 × fINPUT PEAK RIPPLE (5%) IDEAL OUTPUT DC ERROR (0.05%) PEAK ERROR = DC ERROR + PEAK RIPPLE (5.05%) TIME DC AVERAGE OF ACTUAL OUTPUT 1967 F07 Figure 7. Output Ripple Exceeds DC Error C = 0.47µF C = 0.22µF C = 0.1µF 10 INPUT FREQUENCY (Hz) 100 1967 F06 LTC1967 APPLICATIO S I FOR ATIO 0 –0.2 –0.4 PEAK ERROR (%) C = 100µF –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 1 10 INPUT FREQUENCY (Hz) 100 1967 F08 C = 47µF C = 22µF C = 10µF C = 4.7µF C = 2.2µF C = 1µF Figure 8. Peak Error vs Input Frequency with One Cap Averaging 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 = 1.5µF and fINPUT = 10Hz. If the application calls for the output of the LTC1967 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 LTC1967, but the simplest solution is to use a larger averaging capacitor. A 2.2µF capacitor is a good choice for many applications. The peak error at 50Hz/60Hz will be 50kHz inputs. – This is a fundamental characteristic of this topology. The LTC1967 is designed to work very well with inputs of 20kHz 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. 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 (50kΩ) and high accuracy (0.1%) require clean boards! Flux residue, finger grime, etc. all wreak havoc at this level. Solution: Wash the board. KEEP BOARD CLEAN LTC1967 1967 TS09 U 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 LTC1967 output is 50kΩ, resulting in – 0.5% 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 LTC1967 VOUT 5 50k 6 10M DMM 200mVRMS IN –0.5% 1967 TS10 mV W UU DCV OUT RTN 1967f 25 LTC1967 SI PLIFIED SCHE ATIC V+ GND C12 IN1 2nd ORDER ∆Σ MODULATOR IN2 C3 C5 EN TO BIAS CONTROL TYPICAL APPLICATIO S 5V Single Supply, Differential, AC-Coupled RMS-to-DC Converter 5V V+ LTC1967 AC INPUTS (1VPEAK DIFFERENTIAL) CC 0.1µF IN1 VOUT EN 1967 TA02 IN2 OUT RTN GND 26 U W C4 W C1 ∫ C2 Y1 ∫ Y2 C7 C9 OUTPUT + A1 + A2 C8 C11 OUT RTN 1967 SS CAVE – C6 – C10 CLOSED DURING SHUTDOWN 50k BLEED RESISTOR FOR CAVE Single Supply RMS Current Measurement V+ CAVE 1µF DC OUTPUT AC CURRENT 75A MAX 50Hz TO 400Hz T1 10Ω 20k IN1 LTC1967 VOUT IN2 OUT RTN GND EN CAVE 1µF VOUT = 4mVDC/ARMS 1967 TA03 0.1µF 20k T1: CR MAGNETICS CR8348-2500-N www.crmagnetics.com 1967f LTC1967 TYPICAL APPLICATIO S ± 2.5V Supplies, Single Ended, DC-Coupled RMS-to-DC Converter with Shutdown 2.5V ≥ 2V OFF ON ≤ –2V EN DC + AC INPUT (1VPEAK) IN1 V+ LTC1967 VOUT GND 1967 TA04 IN2 OUT RTN –2.5V PACKAGE DESCRIPTIO 5.23 (.206) MIN 0.42 ± 0.038 (.0165 ± .0015) TYP RECOMMENDED SOLDER PAD LAYOUT DETAIL “A” 0° – 6° TYP 4.90 ± 0.152 (.193 ± .006) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 0.254 (.010) GAUGE PLANE 0.18 (.007) SEATING PLANE 0.22 – 0.38 (.009 – .015) TYP 0.127 ± 0.076 (.005 ± .003) MSOP (MS8) 0204 0.65 (.0256) NOTE: BSC 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 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. U U RMS Noise Measurement 2.5V VOLTAGE NOISE IN 0.1µF X7R –2.5V 2.5V V+ + 100Ω CAVE 1µF DC OUTPUT VOUT = VOUT EN 1/2 LTC6203 1k LTC1967 IN1 1mVDC 1µVRMS NOISE CAVE 1µF – –2.5V 100Ω 1.5µF 100k 0.1µF IN2 OUT RTN GND 1967 TA05 –2.5V BW ≈ 1kHz TO 100kHz INPUT SENSITIVITY = 1µVRMS TYP MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660) 0.889 ± 0.127 (.035 ± .005) 3.20 – 3.45 (.126 – .136) 0.65 (.0256) BSC 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 8 7 65 0.52 (.0205) REF 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 1 1.10 (.043) MAX 23 4 0.86 (.034) REF 1967f 27 LTC1967 TYPICAL APPLICATIO R2 1k VIN R1 100k C1 47nF ATTENUATION CONTROL R8 15k V+ R9 10k LT1636 V– RELATED PARTS PART NUMBER LT 1077 LT1175-5 LT1494 LT1782 LT1880 LTC2054 LT2178/LT2178A LTC1966 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 Zero Drift Op Amp in SOT-23 17µA Max, Single Supply Precision Dual Op Amp Precision Micropower ∆Σ RMS-to-DC Converter 2-Channel, 24-bit, Micropower, No Latency ∆ΣTM ADC 20-bit, Micropower, No Latency ∆Σ ADC in SO-8 2-Channel, 20-bit, Micropower, No Latency ∆Σ ADC No Latency ∆Σ is a trademark of Linear Technology Corporation. 28 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 q FAX: (408) 434-0507 q U Audio Amplitude Compressor R5 5.9k ATTENUATE BY 1/4 LT1256 2 C2 0.47µF R3 7.5k R4 2.49k 14 GAIN OF 4 13 V+ – A1 9 1 + 8 R15 47Ω VOUT + A2 7 V– V + VC 3 RC 5 RFS 10 VFS 12 C3 0.1µF V+ R13 3.3k R14 3.3k – R6 2k R7 5.9k – R10 200k C5 0.22µF C4 0.33µF VDD LTC1967 IN1 VOUT OUT RTN IN2 GND EN R12 10k VS = ±5V 0.1µF + 1967 TA07 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) 150µA ISY, 3µV VOS(MAX), 150pA IB(MAX) 14µA ISY, 120µV VOS(MAX), 350pA IOS(MAX) 155µA ISY 200µA ISY, 4ppm INL, 10ppm TUE 200µA ISY, 8ppm INL, 16ppm TUE Dual channel version of LTC2420 1967f LT/TP 0504 1K • PRINTED IN USA www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004