THS4521: Will the protection diode impact the accuracy

Xiaoqiang Shen

Intellectual 420 points

Part Number: THS4521
Other Parts Discussed in Thread: OPA810, THS4561, THS4551, ADS4222

Hello,

I add protection diode D3 and D4.

Will the leakage current of it will impact accuracy?

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Xiaoqiang,

your circuit isn't stable:

xiaoqiang_ths4521.TSC

Also, R42...44 look a bit too big.

What are the protection diodes for? Protection of THS4521 against higher supply voltages of OPA810? Why is D4 is mounted then?

What do you want to drive with the THS4521? And ADC? Which ADC?

Kai

0 Michael Steffes over 4 years ago

Guru 48395 points

Well at some small level they will - that leakage will add to the specified bias currents for the THS4521 where the differential output offset effect will be the mismatch between the total leakage on each side - if your temperature is not very high, should be a very small effect. You don't show your Vocm control voltage, but with only +/-1.6V supplies be careful about its range.

The THS4521 is not a very low offset voltage device to begin with, improved by both the THS4551 and THS4561

0 Xiaoqiang Shen over 4 years ago in reply to kai klaas69

Intellectual 420 points

1) R42...R44 big. --> This is designed as oscilloscope probe.

2) What are the protection diodes for? -> Protection of THS4521 against higher supply voltages of OPA810, yes. When OPA810 output about +/-5V.

3) ADC: ADS4222. using 1.8V power supply.

0 Xiaoqiang Shen over 4 years ago in reply to Michael Steffes

Intellectual 420 points

Vocm comes from ADS4222, it's about 0.95V

0 Xiaoqiang Shen over 4 years ago in reply to kai klaas69

Intellectual 420 points

Why circuit is not stable? What should I change?

0 Xiaoqiang Shen over 4 years ago in reply to kai klaas69

Intellectual 420 points

0 Michael Steffes over 4 years ago in reply to Xiaoqiang Shen

Guru 48395 points

The Vocm loop is a seperate amplifier with its own headroom to supply constraints - the THS4521 requires 1V to the supplies, you are only providing 0.35V so the Vocm loop will not operate correctly - this is from the 3.3V spec table,

+1 Michael Steffes over 4 years ago in reply to Michael Steffes

Guru 48395 points

Normally, these designs use 3.3V single supply on the FDA, that again violates the headroom trying to use a 0.95Vocm - this is why the more recent parts like the THS4551 and THS4561 really tried to move the Vocm headroom closer to the negative supply. I think you need to change parts and then pick this up again.