Document created by analog-archivist on Feb 23, 2016
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### Q

AD9726 DAC output is differential and AD600 signal inputs are not fully
differential. Can AD9726 drive directly AD600 (differential), or do I have to
do converson with OP AMP?
Output of AD600 is not differential (A1CM and A2CM connected to GND). Do you
have reference desing?

### A

The AD600 has a gain range of 0 dB to +40 dB (X1 to X100). If you are using the
AD600 with +/- 5V supplies your maximum output is 6V pk-pk, which is (very
approximately) 2 V rms. The AD9726 delivers 40 mA pk-pk (i.e. differential
outputs with 0-20 mA flowing in each in antiphase) and I assume that you wish
the AD600 to deliver 20 mV to 2 V rms, by variation of its gain control, with
the AD9726 programmed to full-scale digital input. Unless you are working at
the maximum output frequency of the AD9726 you will not need to run the AD9726
at such a high current, but you should not run it at such a low current that
feeding that current into 100 Ohms gives you your 20 mV rms.

You will therefore need to arrange for the AD9726 run at a lower output current
by use of an appropriate resistor on the FSADJ pin and to deliver 20 mV rms to

I cannot see any need for an amplifier here, you have ample signal to spare -
you can do a perfectly adequate job with resistors or with a transformer.

Assume that your programmed total output current is X mA, then the pk-pk output
current is 2X mA and the rms. output current is X/SqRoot2 mA.

The simplest solution seems to me to be to ground one output and allow
X/(2.SqRoot2) mA rms to flow from the other output in a grounded load resistor
R to give 20 mV rms. This calls for R = (40.SqRoot2/X) Ohms. Couple this
voltage to the AD600 input with a capacitor to provide DC isolation (otherwise
the AD600 output will have a DC bias). The value of R may need to be altered to
compensate for the parallel Zin of the AD600 (100 Ohms) but in practice this
error may be trimmed out in other ways.

If you wish the above circuit to be balanced you can terminate the unused
AD9726 output with another resistance of R instead of simply grounding it. NOTE
THAT YOU MUST NOT LEAVE THIS UNUSED OUTPUT OPEN CIRCUIT.

Instead of using unbalanced coupling you can use a broadband transformer with a
centre-taped primary. The centre tap is grounded and the differential outputs
of the AD9726 feed opposite ends of the primary. The secondary is connected to
the AD600 AinHi and to ground. AinLO of the AD600 is also grounded. With this
arrangement it is necessary to shunt either the primary or the secondary of the
transformer with a resistor to prevent the AD600 input signal from being too
large. The value of the resistor will depend on the AD9726 output current
programmed and the transformer ratio.