Other Parts Discussed in Thread: THS4561, THS4551, THP210
I'm using a THS4131 in differential inputs. differential outputs mode to drive a 24-bit SAR ADC with Vref = 5V and Vocm = 2.5V. I want to have the two outputs balanced down to 0.1mV. A picture of schematic is attached. Vref is derived from a reference voltage chip with output current capacity 30mA. I use a Fluke voltmeter up to 4 digit accuracy. I have 2 problems encountered.
1. Vocm is derived from a voltage divider. This Vocm, as soon as, connected to the FDA drops to ~2.42V. If I remove the FDA, the reading is a solid 2.500V. I measured the current drawn from the Vocm pins and its 15mA so it can't be the problem with reference voltage chip not able to provide enough current. Furthermore my simulation on TINA TI also shows this same phenomenon. If I use a set of 500Ohm for the voltage divider, Vocm seems to improve a bit better (about 2.46V) but there not unlimited room for these resistor to keep going down. Why does the FDA cause this problem? Is there any other way to get around this?
2. This is the more important issue. In reality my Vin would be a 100kHz-500kHz, swinging from 0-10V. Hence the 2 differential outputs should swing between 0-5V out of phase. But to test the outputs imbalance/balance, I ground Vin (0V) and I expected to see the difference between Vout- and Vout+ to be -5.000V on the voltmeter. Instead I measured around -4.997V and this is already a big offset for a 24-bit ADC. The set of 4 resistors are 390Ohm 0.1%
I even tried a set of 499Ohm 0.01% and it does not seem to improve and even make it worse.
I know this issue can be attributed by many things such as the PCB layout, but I'd like to know if there is anything I overlooked? How can I improve this outputs balance error?
Any feedback is appreciated! Thank you.