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## AD737JRZ-5-R7 Datasheet(PDF) 15 Page - Analog Devices

 Part No. AD737JRZ-5-R7 Description Low Cost, Low Power, True RMS-to-DC Converter Download 24 Pages Scroll/Zoom 100% Maker AD [Analog Devices] Homepage http://www.analog.com Logo

## AD737JRZ-5-R7 Datasheet(HTML) 15 Page - Analog Devices

 15 / 24 page Data SheetAD737Rev. I | Page 15 of 24Battery OperationAll the level-shifting for battery operation is provided bythe 3½ digit converter, shown in Figure 27. Alternatively, anexternal op amp adds flexibility by accommodating nonzerocommon-mode voltages and providing output scaling andoffset to zero. When an external operational amplifier is used,the output polarity is positive going.Figure 28 shows an op amp used in a single-supply application.Note that the combined input resistor value (R1 + R2 + 8 kΩ)matches that of the R5 feedback resistor. In this instance, themagnitudes of the output dc voltage and the rms of the ac inputare equal. R3 and R4 provide current to offset the output to 0 V.Scaling the Output VoltageThe output voltage can be scaled to the input rms voltage. Forexample, assume that the AD737 is retrofitted to an existingapplication using an averaging responding circuit (full-waverectifier). The power supply is 12 V, the input voltage is 10 Vac, and the desired output is 6 V dc.For convenience, use the same combined input resistance asshown in Figure 28. Calculate the rms input current asOUTMAGINMAGII=µ=++=A125kΩ8kΩ2.5kΩ69.8V10(2)Next, using the IOUTMAG value from Equation 2, calculate the newfeedback resistor value (R5) required for 6 V output usingkΩ48.1μA125V6==R5(3)Select the closest-value standard 1% resistor, 47.5 kΩ.Because the supply is 12 V, the common-mode voltage at theR7/R8 divider is 6 V, and the combined resistor value(R3 + R4) is equal to the feedback resistor, or 47.5 kΩ.R2 is used to calibrate the transfer function (gain), and R4 setsthe output voltage to zero with no input voltage.Perform calibration as follows:1. With no ac input applied, adjust R4 for 0 V.2. Apply a known input to the input.3. Adjust the R2 trimmer until the input and output match.The op amp selected for any single-supply application must be arail-to-rail type, for example an AD8541, as shown in Figure 28.For higher voltages, a higher voltage part, such as an OP196,can be used. When calibrating to 0 V, the specified voltageabove ground for the operational amplifier must be taken intoaccount. Adjust R4 slightly higher as appropriate.Table 6. AD737 Capacitor SelectionApplicationRMS Input LevelLow FrequencyCutoff (−3 dB)MaximumCrest FactorCAV (µF)CF(µF)Settling Time1 to 1%General-Purpose RMSComputation0 V to 1 V20 Hz515010360 ms200 Hz515136 ms0 mV to 200 mV20 Hz53310360 ms200 Hz53.3136 msGeneral-Purpose AverageResponding0 V to 1 V20 HzNone331.2 sec200 HzNone3.3120 ms0 mV to 200 mV20 HzNone331.2 sec200 HzNone3.3120 msSCR WaveformMeasurement0 mV to 200 mV50 Hz5100331.2 sec60 Hz582271.0 sec0 mV to 100 mV50 Hz550331.2 sec60 Hz547271.0 secAudio ApplicationsSpeech0 mV to 200 mV300 Hz31.50.518 msMusic0 mV to 100 mV20 Hz10100682.4 sec1Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.