INA199: two INA199 for one ADC input

Fred Cheng

Expert 4620 points

Part Number: INA199

Hi team,

customer want to use two INA199 and connect their output together to ADC, is that feasible ?

or we have to use analog mux to separate two INA199 output ?

over 1 year ago

0 Javier Contreras4 over 1 year ago

TI__Mastermind 26475 points

Hello Fred,

You cannot do that as you will have both outputs competing and you do not know which output will dominate. How would you determine which current is active? I think the best method is to add a mux.

0 Fred Cheng over 1 year ago in reply to Javier Contreras4

TI__Expert 4620 points

HI Javier,

how low should the mux Ron be?

may I also know the Rout of INA199 since it might be affected by the mux Ron?

Regards,

Fred

0 Javier Contreras4 over 1 year ago in reply to Fred Cheng

TI__Mastermind 26475 points

Hello Fred,

Ron could be high and may be dependent on the ADC you are driving. So if you are driving a SAR ADC and you need fast sampling that would be a different consideration. See https://www.ti.com/lit/an/sboa443/sboa443.pdf . If it is a standard measurement and DC the output impedance at DC is in the 100mΩ this can be seen by the output swing vs output current but you normally don't need that much DC current. The Ron may impact you more on the measurement side as the impedance of your measurement will create a divider between your Ron and R of your measurement instrument.

0 Fred Cheng over 1 year ago in reply to Javier Contreras4

TI__Expert 4620 points

Hi Javier,

it's connect to ADC instead of measurement which mean we only care about RC time constant , but thanks for the elaboration.

how about the output impedance of INA199 since it's an OPAMP,

or it's ideally 0 ohm?

Regards,

Fred

0 Javier Contreras4 over 1 year ago in reply to Fred Cheng

TI__Mastermind 26475 points

Hello Fred,

The output impedance from some measurements I saw at DC is under 100mΩ.

0 Fred Cheng over 1 year ago in reply to Javier Contreras4

TI__Expert 4620 points

HI Javier,

thanks for your reply,

I wonder why this parameter is not listed in datasheet?

I mean Rout

0 Javier Contreras4 over 1 year ago in reply to Fred Cheng

TI__Mastermind 26475 points

Hello Fred,

That numbers is typical for most amplifiers at DC. Swing High and Low vs Current is in the datasheet which is normally called the claw curve give you some insight on that. Since the release of this we have added the Zout vs Frequency for newer components.

0 Fred Cheng over 1 year ago in reply to Javier Contreras4

TI__Expert 4620 points

Hi Javier,

sorry,

would help me briefly go through how these curve could give me the information of Rout= 100mohm ?

0 Javier Contreras4 over 1 year ago in reply to Fred Cheng

TI__Mastermind 26475 points

Fred,

I am sorry you are correct. This is for max current an the output stage saturated. This gives you the saturation point.

0 Fred Cheng over 1 year ago in reply to Javier Contreras4

TI__Expert 4620 points

HI Javier,

so let me conclude this,

so datasheet doesn't mention Rout, but 100mohm is typical and common for most of the OP in TI,

please correct me if I'm wrong.

customer already put a 1K resistor right after the OP output , so I guess the Ron of analog mux is not the bottleneck here...

0 Javier Contreras4 over 1 year ago in reply to Fred Cheng

TI__Mastermind 26475 points

Hello Fed,

That is correct. But this also depends what you are driving. That is why when driving a SAR ADC you may need a buffer. I would say a 1k at the output will not be an issue if measuring it slow. The document I referenced has more detail on this.

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INA199: two INA199 for one ADC input