AD5791 circuit feedback

Hi ultraspeed ,

Our expert   is currently checking your query.

Best Regards,

Den

ultraspeed 

Schematic overall looks OK. A few suggestions

• small value capacitor can be added at the output of the reference buffers to gnd. can be unpopulated and will be useful for debug. 
• on the output of U50 (pin6) a series R, small footprint, can be added to have a possible in-the-loop compensation if needed. 
• not a big fan of adding pull-up resistor for SDIN and SCLK, these pins should be driven with CMOS logic output making pull-ups unnecessary. pull-ups on CLR, SYNC, RESET and LDAC are OK. 

What application are you using the AD5791 for?

Best regards,

Ian

Hi  dendalisay 

Thank you for the detailed suggestions. This circuit is already fabricated and we can de-solder pull-up resistors on SDIN and SCLK. Does it have any effect on its performance?

After testing this circuit, I found when the DAC is powered on and RESET, LDAC and CLR pins are low, the output voltage is 1.353 V, whereas the datasheet mentions the output should be VREFN in this condition since the DAC output is in tri-state. Below is the initialization sequence I'm following - 

1. Power up DAC (VDD, VCC and IOVCC are turned on at the same time). *DAC output 1.353 V*

2. Pull RESET, LDAC and CLR low. Also, pull all other SPI pins low. *DAC output 1.353 V*

3. RESET = 1, CLR = 1, LDAC = 0.  *DAC output 1.353 V*

4. Disable DAC by writing 0x20000E. *DAC output 1.353 V*

5. Confirm DAC disabled by writing 0xA00000 to read DAC register. *DAC output 1.353 V*

6. Enable DAC by writing 0x200002. *DAC output 1.353 V*

7. Confirm DAC enabled by writing 0xA00000 to read DAC register. *DAC output 1.353 V*

8. Write 0x180004 to DAC to set DAC output voltage to 0 V.  *DAC output 2.08 V*

9. Confirm DAC value by writing 0x900000 to read DAC register. *DAC output 2.08 V*

Could you please suggest any changes in the initialization sequence.

I measured voltages on Pins 3, 4, 16 and 17.

Pin 3 VREFPS = -8.65 V

Pin 4 VREFPF = -1.93 V

Pin 16 VREFNF = 6.1 V

Pin 17 VREFNS = 8.6 V

The root cause of the problem is AD8676 (U52) pins 5 and 3 are swapped.

Pin 3 should be connected to VREFN

Pin 5 should be connected to VREFP