AD1992: Driving the input stage

Document created by analog-archivist Employee on Feb 23, 2016
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I would like to control the current (if not possible then the voltage)running
through the 2 voice coils by controlling both single ended inputs of the AD1992
using a DC coupled (no capacitor) AD5668 DAC.

- Is it possible to drive the inputs of the AD1992 directly with the AD5668 (it
looks OK since imput impedance of AD1992 is 20K and the DAC can source <30mA).
- What are the limitations for R and L of different voice coils I can use (to
ensure stable operation of the AD1992)?


The input buffer of the AD1992 is an inverting amplifier with the non inverting
(+) input connected to Vdd/2, nominal. Therefore, to get zero output you need
to get the DAC to output Vdd/2. However the precise voltage used to bias the
input stage may have quite a large tolerance associated with it (+/-10% or
more) therefore it will require some adjustment to get the zero point exactly
right. Other than that the AD5668 should be able to drive the AD1992 with no
problems. It is an octal DAC - do you also need 8 channels of Class-D amplifier
i.e. 4 units?

Besides that, there are very few specifications relating to DC precision, so if
DC precision is important in your application you may have some problems with
the precision of the AD1992.

The stability of the AD1992 depends more on the gain settings (see page 14 of
the datasheet) than on the nature of the load.

If the output current exceeds 4A then the output protection will begin to
operate, so the load impedance and Transistor power supply should account for
this output current limitation.