AD9726: Interface with the AD600

Question 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? Answer 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 the AD600. 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.

AD9726_Iout equation error

Question I have a question about the relationship between IOUA and IOUB of AD9726. For IOUTA, 0 mA output is expected when the inputs are all 0s. For IOUTB, 0 mA output is expected when all inputs are set to 1. But the equations as shown in page 19, IOUTA = IOUTFS × DB[15:0]/65536 IOUTB = IOUTFS × (1 − DB[15:0])/65536 are not corresponding to the results. Answer The equation in the AD9726 data sheet is incorrect. They should be: IOUTA = IOUTFS * DAC_CODE/65536 IOUTB = IOUTFS * ((65536 – 1) – DAC_CODE)/65536

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AD9726 Not Recommended for New Designs
The AD9726 is a 16-bit high performance LVDS DAC that offers exceptional noise and spurious performance and true 16-bit dc linearity. Manufactured on a...
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AD9726 on Analog.com