How to use LMP7721 to create a logarithmic current-to-voltage converter?

Use LMP7721 to create a  logarithmic current-to-voltage converter. Can I use a diode to replace R1.

But if consider the effect of temperature, use triode to replace is better or not?

Amy Luo：

Hi,
Thank you for using e2echina forum.

What is the current range you need to convert? You can also choose TI's current sense amplifiers ：
www.ti.com.cn/…/slyb194d.pdf

TI has the special logarithmic amplifier, please refer to the selection link:www.ti.com.cn/…/products.html

user6354633：

回复 Amy Luo:

Hi，thank you for your answer.
The current range is 10pA~1mA.

Amy Luo：

回复 user6354633:

Hi，
The logarithm operation circuit realizes the logarithm function between the output voltage and the input voltage, and cannot convert the current into the voltage. The purpose of replacing diode with triode is to enlarge the dynamic range of input voltage.

The current of pA level requires that the input bias current of op amp is less than the detection current.Can you give me more details: What are you measuring the current of? What is the system load? what kind of output range do you need?

user6354633：

回复 Amy Luo:

Hi，

1.The application environment is Molecular electronics, so the current will change form fA to mA(10^-11A~ 10^-3A).

from the datasheet of LMP7721, the input bias current is  ±20 fA at 25°C, it can meet the requirement .

2. we used OPA129 before, but OPA129 was discontinued product, so we want to use LMP7721 to replace, it's OK?

3.the output range is  0~5V .

thank you!

Amy Luo：

回复 user6354633:

You can try to make a trans-impedance amplifier. Transimpedance amplifiers can convert current into voltage.