AD633: Asymmetric supplies

Document created by analog-archivist Employee on Feb 23, 2016
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I would like to know whether it is possible to use a AD633 analog multiplier
with unbalanced supplies eg +24v,-5v or +12v,-5v

Alternatively can you recommend a way to calculate a square of a voltage in a
single supply circuit or one with a very limited negative supply.

 

It will not be possible to use the AD633 with a supply of -5v on the negative
side as this does not provide enough headroom for operation.

The problem with making low voltage analog multipliers is that they have a very
large dynamic range, because both inputs can vary from very small signals to
very large ones. For example with the AD633 you have a transfer function of W =
XY/10 + Z. X and Y have maximum offsets of +/-30mV, and maximum values of
+/-10V at +/-15V supplies so you have a dynamic range of 0.03*0.03/10 to
10*10/10 which is 0.0009V to +10V or a dynamic range of 10000 = 80dB.

In reality, the dynamic range is limited further by the output offset voltage
and the noise. Reducing the supply voltage from 30V (+/-15V) to 5V  or 12V
makes it very difficult to achieve the same dynamic range.

A alternative is to consider the AD8361 which has a wide bandwidth and offers a
true rms reading for different waveforms (sine, square, noise-like) on +3V or
+5V single supply. The ADE7758 (an energy metering device) can also perform
this function in the digital domain.

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