INA233: Device Selection

McKenna Strambi

Intellectual 2280 points

Part Number: INA233
Other Parts Discussed in Thread: INA333,

Hi expert,

My customer has an application in a tester to measure current for power consumption at both active and idle.

Active current is ~70mA

Idle current is ~10uA

Voltage is ~5V.

They have typically used current sense amp like INA19x device in the past.

However, someone suggested to use instrument amplifier such as INA333 instead.

Questions:

Can you provide some feedback about current sense amp vs instrument amp
Looking at INA333 datasheet, the input offset is +/-10 +/- 25/G uV. Does this mean if Gain is set to 1000x, the input offset is +/- 10 +/- 25000 uV ~ +/- 25mV?

Thank you!

McKenna

2 days ago

0 Keith Barkley 2 days ago

Guru 42335 points

Is this a high side test?

You might run into Common Mode range problems. Note that the Vcm is V+ - 0.1 V.

0 McKenna Strambi 2 days ago in reply to Keith Barkley

TI__Intellectual 2280 points

Hi Keith,

The instrument amp configuration is currently low side but they are open to making it high side. Which would be better?

Thank you!
McKenna

0 Keith Barkley 2 days ago in reply to McKenna Strambi

Guru 42335 points

Neither. Look at the Common Mode spec. It does not reach to either rail. (Unless your V- is negative)

0 Keith Barkley 2 days ago in reply to McKenna Strambi

Guru 42335 points

I am being a bit short with you. If the supply to the tester is +/- 12 V, you can easily use it to measure the voltage across a current-sensing resistor in a 5 V system. It might be easier to use a high side sensor to avoid a resistor in the ground path.

0 Robert Clifton56 2 days ago in reply to Keith Barkley

TI__Mastermind 28740 points

Hi McKenna,

McKenna Strambi said:
Can you provide some feedback about current sense amp vs instrument amp

Excellent question. We actually have a piece of content called Difference Between an Instrumentation Amplifier and a Current Sense Amplifier that will be able to answer this for you. It goes over the tradeoffs that Keith was mentioning as instrumentation amplifiers tend to have much more limited common mode voltage range compared to current sense amplifiers.

McKenna Strambi said:
Looking at INA333 datasheet, the input offset is +/-10 +/- 25/G uV. Does this mean if Gain is set to 1000x, the input offset is +/- 10 +/- 25000 uV ~ +/- 25mV?

Not quite. I always get a little confused by the calculations here too. When looking at the output referred error, multiply the input formula by the gain. This would read out like Vos = G* Vos1 + Vos2 where the first stage is Vos 1 and the second stage is Vos2. High gains cause the first stage offset to dominate the system while in low gain settings, the second stage usually dominates.

For a gain of 1000V/V, Vos = 10*1000+25 = 10mV.

Best Regards,

Robert Clifton

0 McKenna Strambi 1 day ago in reply to Robert Clifton56

TI__Intellectual 2280 points

Hi Robert,

Thanks for your inputs here! I'm trying to understand what's the input referred offset at 1000x gain, not output. Is this a typo? Still not sure how to read +/-(25/G) in this datasheet. Below is a screenshot of the datasheet the customer sent - though it looks old.

Here is the new datasheet - is this a typo in the older revision or am I misunderstanding the calculation?

Thank you!

McKenna

0 McKenna Strambi 1 day ago in reply to McKenna Strambi

TI__Intellectual 2280 points

Hi Robert,

Actually, I pasted the INA233 and not INA33. My apologies. They are still confused on how to read the +/-25G. Can you help clarify?

Thank you,

McKenna

0 Keith Barkley 1 day ago in reply to McKenna Strambi

Guru 42335 points

The "G" is the gain as set by the Rg resistor, so the formula is +/-25 + +/- 75 / Gain. [75 divided by gain ] Where Gain is 1 + (100K/Rg)

0 Robert Clifton56 1 day ago in reply to Keith Barkley

TI__Mastermind 28740 points

Hi McKenna,

Looking at the simplified block diagram, we have two stages. A1/A2 and A3.

When calculating what the offset voltage on the output the formula would be as shown below:

Only the first stage is affected by the gain of the system, while A3 will be unaffected.

To determine the offset voltage referred to the input, you divide the equation by the gain G. That's how we derive the formula shown in the datasheet.

Best Regards,

Robert Clifton

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INA233: Device Selection