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TRUE RMS-TO-DC Adapter For DVM
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.
Specifications
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
Introduction
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 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.
Construction
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.
Find updates at www.projects.cappels.org
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First posted in June, 2011
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