Need a switch

Feb 06, 2006 2 Replies
D

I am using a computational unit from Analog Devices (AD538) to divide the outputs from two rms to dc converters (AD637). The objective is to real time monitor the constantly changing output impedance of a purely resistive circuit operating at



400 kHz. The dc equivalent of the rms voltage is the numerator and the dc equivalent of the rms current is the denominator in the calculation. The resulting division from the AD538 is monitored and recorded with an o'scope.

My problem is that the output of the circuit being monitored is frequently cycled on/off. Anytime the circuit is "off", my denominator (current) is zero (or near zero with noise), so I end up with "divide by zero" infinite oscillation on the output from the AD538.



I could probably live with this if I had to, but my preference would be to ground or turn off the output of the AD538 whenever the circuit being monitored is off or whenever the circuit's current falls below about 100 mA. The AD538 has a +/-15V supply.



Any suggestions on how to accomplish this?



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S

A comparator driving analog switch should do the trick. AD has some switches that will work on +/-15 supplies.

You could also use a simple op-amp precision clamp circuit to limit how close to zero the denominator term goes.

Best regards, Spehro Pefhany

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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T

In article , wrote: [snip]

ASCII-art below. --+-- +Vsupply | R1 | +----------------->Vout | D +----|-----+ Vin------|+_/ | R2 | 0v-----+----------------

It's a unity-gain follower, but Vout(min) equals the voltage across R2. For a 10V swing, R2 cannot be much less than 2K and this is the output resistance when Vout = Vout(min).

Tony Williams.
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