INA169: ina169

Paolo Motta

Prodigy 90 points

Part Number: INA169
Other Parts Discussed in Thread: REF200, , OPA340

Hi i need help with math for this circuit please verify my calculation

INA Power supply 5V

Vin 9 to 36 V

I = 0.1A to 5A

Vout Max = 3v3

I use INA169 , REF200 for offset and OPA340 as buffer (out of OPA to PIC microcontroller).

From my calculation a Rs of 10 mOhm Vs=50mV

gain required 3v3/0.050 = 66 so i choosing 50k (2.5 at Imax)

But i don't understant Offset:

I think 10uA x 50k = 0,5V

So out at Imax is 0,5 + 2.5 ???

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Paolo,

the 10µA, what is it? The input bias current? This will flow across the 10mR shunt. Then you must also take into account the input offset voltage. Please also see its footnote on page 6 of datasheet.

Kai

0 Paolo Motta over 6 years ago in reply to kai klaas69

Prodigy 90 points

Hi Kai
10uA is the offset current from REF200. In the datasheet of ina is a way for offsetting output in case of zero load and Adc single power supply (in my case Adc from pic micro)

0 Patrick Simmons over 6 years ago in reply to Paolo Motta

TI__Mastermind 24075 points

Hello Paolo,

Thanks for considering to use Texas Instruments. As I understand, your system looks like the diagram below. I think you meant to say that the REF200 sources 100uA. Your output going to the OPA340 can be calculated by the formula below. Calculation yields that to be 7.5V. You will likely need to reduce your load resistor or change which reference you are using.

0 Paolo Motta over 6 years ago in reply to Patrick Simmons

Prodigy 90 points

Thanks Patrick for reply,
All you say it's ok... schematic is same
For now i need to calculate the correct value for output and you say it's 7,5V. it's very high, i think to insert voltage divider at the output of OPA or change value of Rl. Thanks for the support

0 Paolo Motta over 6 years ago in reply to Patrick Simmons

Prodigy 90 points

Hi Patrick,

you insert opa340 in inverting mode. But i think output of INA need to connect to + terminal of OPA and - of OPA connect to output of OPA. see image

0 Patrick Simmons over 6 years ago in reply to Paolo Motta

TI__Mastermind 24075 points

Hello Paolo,

Thank you for pointing that out. I made a mistake in my previous post. Your figure is correct.

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INA169: ina169