INA826: If input put the high resistance (1M) to GND.

Hey Jimmy, 

In order to assist I would need to know the circuit schematic, and details such as input common mode voltage, supply and ref pin voltage as well as the gain. Could you provide a complete schematic? We often use 1M to ground for biasing input common mode to 0V in other applications so I would need more details to fully understand the situation.

Best Regards, 

Chris Featherstone

Hi team,

FYI

Hey Jimmy, 

I appreciate the schematic however I am unclear as to where the 1M resistors are? 

• Can you show where the 60V is getting divided down? is it at pins 5 and 12 of the relay?
• Where are the 1M ohm resistors on the schematic?
• What is the relay used for?

There is a common mode limitation of the INA devices. In the engineers calculator I have put in the gain, supply, reference voltage and common mode voltage at 0V. 

https://www.ti.com/tool/ANALOG-ENGINEER-CALC

You can see the common mode voltage vs Vout. Below. Is the common mode voltage being kept within the linear operating bounds shown below?

Best Regards, 

Chris Featherstone

Hi Chris,

Can you show where the 60V is getting divided down? is it at pins 5 and 12 of the relay?

A:This is when the customer expects a voltage of 60V but has not yet measured it. The OP output is not 0.

• Where are the 1M ohm resistors on the schematic? A:=> My fault. The R is 2M.
• What is the relay used for? A: If customer wants to measure the DUT. will turn on the relay.

Hey Jimmy, 

Thank you for the clarification. Has the customer considered a device such as the INA169 as shown below? The input common mode of this device can be 60V. I assume this is a high side current sense application and the 200 ohms is the sense resistor. The input common mode voltage of the INA826 is limited the voltage range shown in my last post. If my understanding is correct the customer may benefit by using one of the devices shown in the links below. 

https://www.ti.com/lit/ds/symlink/ina169.pdf?ts=1714733229604&ref_url=https%253A%252F%252Fwww.google.com%252F

List of current sense amplifiers with high input common mode voltage. 

https://www.ti.com/amplifier-circuit/current-sense/products.html#726max=48%3B120&sort=726max;asc&

Best Regards, 

Chris Featherstone

Hi team,

So will it cause problems if I put a voltage divider resistor on the input? if use INA826.

Hey Jimmy, 

I have analyzed the setup in Tina Ti along with our calculator to demonstrate the issue. Below you can see that dividing down the 60V with the voltage divider sets the input pins beyond the min and max values for linear operation. The output is railed out as well. 

I can further divide down the voltage potential as shown below however this comes at the expense of large resistors and the CMRR will be compromised due to resistor tolerance mismatch between each input. 

For these reasons I recommend using a device that can handle 60V common mode such as the INA169 previously mentioned. We also offer INA148 and INA49 that have very high common mode voltages. 

https://www.ti.com/lit/ds/symlink/ina148.pdf?ts=1715118153629&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/ds/symlink/ina149.pdf?ts=1715118236154&ref_url=https%253A%252F%252Fwww.google.com%252F

INA849 application circuit. 

6866.INA826.TSC

Best Regards, 

Chris Featherstone