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Hello,
We are looking for current sense circuit with overall tolerance of 1% to 2% and we are planning to use INA169 / INA28x / INA225. I need error budget for INA169 with Vsense=300mV input, VCM=24V, V+=5V and RL=~25K ±0.1%
1) As per datasheet of INA28x, which is a difference amplifier, error budget is as follows. INA169 is a current output device and what are the errors to be considered for INA169.
2) As per one of the application note, bias current will introduce error if equivalent impedance at + / - input of op amp is different as shown below. What will be the error due to 1Kohm internal input resistance, 5 ohm sense resistor and 10uA bias current for INA169?
Thanks & Regards,
Chethan K S
Hello Chethan,
Thank you for your interest in using TI with your design. As for determining the input bias error, you can use the diagram below. With that and the details you provided, I did some calculations on what I would expect there error to be. I assumed you were using a 5 ohm shunt with 0.1% tolerance.
Hello Patrick Simmons,
Thank you for providing the required information. I need few additional details for worst case analysis.
1) As per your diagram used for error estimation, bias current of 10uA (typical value as per datasheet) will not flow through the internal 1K ohm resistances. What will be the bias current of front end amplifier which will flow through the 1K ohm resistors to its (+) and (-) inputs?
2) eIB calculated above assumes identical internal resistance and identical bias current in same direction for both (+) and (-) inputs of of front end amplifier.
As per section 7.1 of INA169 datasheet, internal resistors are trimmed to an absolute tolerance. I am assuming <0.05% absolute tolerance (<0.1% relative). Please let us know if exact specification is available.
As per FAQ "How Vos and Ib affect small current measurements" Figure 4, bias current can be different and even difference polarity w.r.t differential input voltage. INA169 datasheet provides 10uA typical bias current. I am assuming <20uA as maximum bias current. Please let us know if exact specification is available for front end amplifier.
With Internal resistance difference RG2 - RG1 = 0.1% x 1000 Ohm = 100 Ohm.
With maximum bias current difference (IB+ ~ IB-) of 10uA for front end amplifier, offset error due to bias current is 1mV.
Is my calculations correct?
This will add 0.33% at 300mV input and 1% at 100mV input?
Thanks & Regards,
Chethan K S