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This uses an Analog Devices AD737 converter chip to make a battery powered
True RMS adapter for DVMs.


  Shown while reading a 2 volt P-P sine wave.
Output voltages are always negative.

Input: AC, DC or AC+ DC to > 10 KHz
Ranges: 200 mv, 2V, 20V, 200V, 600V full scale
Accuracy: ±1%, depending on divider resistor selection
Crest Factor: 1 to 3, up to 5 with degraded accuracy
Input impedance: 1 Megohm shunted by 20 pf
Battery drain < 300 uA


The DVMs in the photo above measures AC voltage by detecting the peak AC voltage on the 200 VAC or 500 VAC scale, then assumes that the input wave form is a sine wave and scales the meter reading to 0.707 of the peak. It is not useful for measuring waveforms that are not sinusoidal. When I found the AD737 at my favorite distributor for only 135 Thai Baht (Approx. US$4.40 today), I bought a couple, and only adding a few other components, made this adapter for my voltmeters.

The Circuit

  The circuit is based on an application example in the Analog Devices AD737 data sheet.

The main differences between one of the application circuits in the AD737 data sheet are the addition of a MCP1702 micropower 3.3 volt regulator to split the power supply and two 11 volt zener diodes to prevent the voltage across the chip to become high enough to damage the chip in case the input is connected to an excessive voltage while in the 200 mv full scale setting.

The input divider was salvaged from an inexpensive DVM in which the mode selector switch wore out. The resistor values are all standard 1% values and even if the desired values are not available, you can get to within a percent by using series/parallel combinations of other 1% values.

The AC input coupling capacitors are two film capacitors rated for "X" usage (across the power lines), being rated at 300 volts each, the capacitors can safely handle an offset of over 800 volts DC, but the resistor divider probably can not, so I decided to limit use to under 600 volts DC.

The input impedance of 1 Megohm shunted by 20 pf was selected to match that of my Tektronix oscilloscope so that I can confidently use probes and other signal sources designed for sue with the scope. For this purpose, I added a BNC connector in parallel with the banana jacks at the input.


The circuit was made in two subassemblies. One is the resistor divider and the 3.3 volt power supply mouthed on the back of the two pole, six position rotary switch.

Do not try to use this as a wiring guide because it
contains a mistake that
was found and corrected after this photograph was taken.

The resistors are splayed out to reduce parasitic capacitance.

Once the switch subassembly was completed, tested, and corrected, I mounted the switch assembly and banana jacks on the inside of the cover of a project box and proceeded to make an "L" shaped circuit board to support the remaining components. The board was mounted copper side up. The board has pads around holes drilled every 2.54 mm. The AD737 integrated circuit is in an SOIC-N package, and it was soldered directly to the copper pads, some pins being bent up, away from the board because the pads are too far apart. Analog Devices also makes the AD737 available in a plastic DIP package, which would make it much easier to use in a hand wired assembly. 

Everything except the BNC connector was built onto the lid of a plastic project box.
A small 3 pin connector is used to connect the BNC connector to the input.
The 9 volt battery is held in place with a piece of foam rubber.
One of the smaller surface mount capacitors on the board is
 not used because of a change made after initial assembly.

Performance And Use

I found that the readings correspond closely with the calculations of my ancient Tektronix TDS2002, when on the 200 mV and 2 V scale, I connected both to my function generator  and switched between sine, square, and triangle wave forms and varied the duty cycle of each. I varied the frequency from a few Hz to over 10 kHz. Errors of several percent were observed on sine waves at about 10 kHz and above.

I also connected it to my 1.024 volt DC voltage reference and noted a reading of 1.023 volts on the meter.

The circuit works well when driven by a Tektronix P2220 switchable scope probe, making it pretty handy. I can use the probe in the X10 position and then only load the circuit being observed with 10 Megohms.

One unfortunate though minor problem with the AC coupling is that when in the AC coupled mode, the adapter outputs an approximate 1.5 millivolt offset.  I have not been able to determine the source of this offset.

Another unpleasant, though anticipated aspect of operation is the very long settling time. This can be shortened, but at the cost of accuracy with lower frequency inputs. See the Analog Devices AD737 data sheet for a discussion of these tradeoffs.

The current drain from the 006P 9 Volt transistor radio battery is less than 300 microamps, and is so low, that I did not bother to include a battery test button; I figured that if I pressed the test button, the battery test circuit would draw more energy from the battery than would the adapter circuit.

A nice improvement might be an externally accessible test point that can be used to see the waveform presented to the AD737 so that scope probes can have their compensation adjusted while connected to the RMS-to-DC converter.

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Contents ©2011 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in June, 2011

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