INA303: INA303--Common-mode voltage is a bit small，How to improve the design scheme

aiwen chen

Part Number: INA303
Other Parts Discussed in Thread: INA138

1、It mainly does DC sampling, the front stage is a sampling resistor, and then detected by INA303, but the common mode voltage of INA303 is a little small, only 36V, the common mode voltage of the front input is as high as 120V, I want to string another 1KΩ resistor between the sampling resistor and INA303 to reduce the common mode voltage, is this scheme feasible, or is there another sampling method? Or other models recommended?

over 1 year ago

0 Peter Iliya over 1 year ago

TI__Mastermind 29055 points

Hello Aiwen,

This is not a recommended, nor valid option with this device and most other current sense amplifiers (CSAs). CSAs are low input impedance and so adding input resistors that are > 10-Ohm will significantly alter device error. Even when you compensate for the new circuit gain when taking into account the input resistor network (which is not shown in the image you posted), you can still a wide range of gain error because we do not trim the internal resistors to accurate absolute values, we only trim the ratio of the values to get the accurate gain error. Also the the 1kOhm would not provide enough current limiting to protect the device from the violating 120Vcm and it would burn up.

There are a few options.

1. Use input resistor divider at input pins to divide down Vcm (not recommended). Even with input resistors that have tight tolerance, you will still get a large gain error range due to +/-20% internal resistors inside device. See the link for our training video covering this. You will also burn power through the resistor dividers as well.

https://training.ti.com/input-filter-error

https://www.ti.com/lit/ug/tidu849/tidu849.pdf?ts=1668023375331&ref_url=https%253A%252F%252Fwww.google.com%252F

2. Choose an amplifier that has an operation Vcm rating up to 120V.

https://www.ti.com/amplifier-circuit/current-sense/products.html#p726max=120;120

3. Create a floating Zener voltage regulator that is biased off the 120V bus. Then level-shift, or isolate the output signals to your low voltage circuitry. See the following circuits for ideas on this. The first link is using a current-output amplifier (INA138), but you could convert the INA303 into a current-output device using the following Howland Pump circuit also provided below (in the second link).

https://www.ti.com/lit/ug/tidu833/tidu833.pdf

https://www.ti.com/lit/an/sboa358/sboa358.pdf?ts=1619563437533&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/ug/tidu849/tidu849.pdf?ts=1668023375331&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/ug/tidu361a/tidu361a.pdf?ts=1668023201058&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA226

https://www.ti.com/lit/an/sboa198a/sboa198a.pdf?ts=1668023180097&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA226

Sincerely,

Peter