OPA328: The guard of charge AMP by using OPA328

Bing Zhao

Part Number: OPA328
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hi team,

i design a charge AMP by using a discrete JFET and OPA328,i know that a guard is necessary to surround the input in PCB to prevent Leakage current .but the circuit contains a JFET makes me confused about the guard ，which node can be the guard ？On the other hand ，any advise about the circuit？ Please advise and let me figure it out , looking forward to reply.

Regards

3 months ago

0 Marek Lis 3 months ago

TI__Guru 64840 points

Please attach your Tina-TI schematic within this post.

0 Bing Zhao 3 months ago in reply to Marek Lis

Prodigy 40 points

Hi Marek,

I upload the schematic as below ,thank you .

Isbomc38 - autosave 24-11-20 15_48.TSC

+1 Marek Lis 3 months ago in reply to Bing Zhao

TI__Guru 64840 points

Bing,

In order to minimize the error caused by a leakage current, high impedance nodes must be surrounded by a guard ring. Simulating the leakage of 1nA from various nodes shown below, it is clear that the only node sensitive to a leakage current is shown in Fig 2. Thus, the entire red area around said node must be surrounded by a guard ring.