AD8203 cross reference replacement

Hi Mauro,

Thanks for your post, I can help you. 

Looks like you're searching for a low voltage instrumentation amplifier for a cost sensitive application. I definitely want you to check out INA317 (www.ti.com/.../ina317.pdf) and help me answer the following questions to make sure you are operating linearly:

• supplies? 5V single supply
• input differential voltage range?
• expected output?
• common mode voltage? (at any instant, what do you see on -IN and +IN?) 

Hi Tamara

Thank you for your answer

• supplies? 5V single supply - better 3,3V single supply
• input differential voltage range? The input of the circuit could be 0-10V or 4-20mA, with our circuit we have 0,66V on the differential input with Gain 4 to obtain 2,64V on the output. So if we want the same output with gain=1 the input must to accept 2,64V but I think that better solution could be a differential input of 2V with gain 1,33 (setted by Rg) to obtain 2,66V on the output 
• expected output? 0 - 2,66V for us is very important to have a low swing to GND
• common mode voltage? (at any instant, what do you see on -IN and +IN?) the differential input is an option, usually the input is used like single ended so in - IN we usually have 0V and on +IN we can have 10V. For us is important to read an input of 0V with -IN referred to 0.

  Thank you

  Mauro

Hi Mauro,

I put together a suggested schematic for you. A few comments:

• On a 3.3V single supply operation, you will not be able to get 0V on the output because no amplifier can swing that close to the rail on the output. The INA317 output swing on a 3.3V single supply can only be as low as 0.3V and as high as 3.3V. If you would like to capture 0V at the output, you would need a negative supply rail.
• The same rationale also applies to your input. You mentioned that IN- would likely be ground, and that IN+ would see 0V to 0.66V.
• Instrumentation amplifiers are selected for their ability to amplify the differential voltage. If you are intending to always have a single ended input signal, I would recommend a much simpler solution: operational amplifier with feedback resistors to set your gain. 

Below is a TINA file of a suggested circuit with the INA317:

INA317_Mauro.TSC

Below is a TINA file of a suggested circuit with the OPA333 and feedback resistors:

OPA333_Mauro.TSC

Hi Tamara

Thank you for the circuit, really I'm serching for an instrumentional amplifier because the resistors for the feedback are inside the component, so our application aren't influenced by the tollerance of the resistors(I'm speacking about mass production).

I' know that is not possible to reach 0V without negative supply but I've seen that the swing to GND of the INA317 is 50 mV, why do you speak about 0,3V min on the output? 

Thank you

Mauro