谢谢您的答复，我发现ADA4075-2好像不行，它是高压放大器；我们选用了AD8613，电路图如下所示，是否正确，期待您的答复

Hi Robert,

We have reference circuit for high impedance sensor as shown below though it uses opamps instead of inamps.  Opamps use for this type of applications are AD8603, ADA466,1 AD4505 and AD8613.  

CN-0326 - Isolated Low Power pH Monitor with Temperature…

Carlpiro,

As Kaung said, it's best if  you can run the AD820 on 5V min.  I'm retired from ADI, but used to be in the op amp group.

The problem with JFETs is  the two to one or three to one variation in Vp and IDss.  If you look at the schematic…

你好,如果你要放大的信号不是差分信号的话，就选用一般的通用放大器就可以了，比如：AD8613, AD8538都可以用单电源5v供电，也可以满足带宽要求。

Klaus san

Thank you for your reply, and apologize for that reply becomes slow.

I drew the easy circuit diagram in the attachment.

In the case of ADG5208F's VDD : 24V, I think it has Ron about 450 ohm (typ.).

So it current is 5V/ 450ohm = 11mA…

你好，推荐使用AD8603,这款放大器的最大偏置电流及最大输入失调电流分别为1pf和0.5pf,可以满足测量1nA电流的需要。其它的放大器还有AD8605，AD8607，AD8609，AD8613，AD8615，AD8627等也可满足你的应用需求。I-V转换电路可以参考本论坛中西安交大知名教授杨建国最新力作——《你好，放大器》（电子书下载地址http://h.analog.com/China_ebook_hello_amp）中的161页运放检测电路方案。

参考电路图如下：