INA240EVM: Maximum capacitive load

Danilo A.

Part Number: INA240EVM
Other Parts Discussed in Thread: INA240, TINA-TI

Hi Team,

The datasheet of INA240 states the maximum load capacitance is 1nF. However, a customer wants to add a low pass filter by connecting a capacitor on the output. Based on the simulation, there are issues of adding capacitor on the output of INA240. The customer wants to know why the maximum load capacitance is relatively low?

Regards,

Daniloa

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Danilo,

the best way to add a low pass filter to the output is to provide an RC filter. A simple C, on the other hand, destabilizes the output of INA240 by eroding the phase margin. No good idea...

Kai

0 Carolus Andrews over 3 years ago in reply to kai klaas69

TI__Mastermind 18790 points

Danilo,

Kai is correct here, and this phenomenon is true for most amplifiers. Amplifiers in general do not like to drive capacitive loads due to the interactions with the output impedance of the device. In most cases though, a resistor placed between the output impedance and the capacitor (forming an RC filter as Kai mentions) will alleviate the issue.

0 David Philpott over 3 years ago in reply to Carolus Andrews

Prodigy 150 points

I currently have an RC filter on the output of the INA240 with a series R of 300-500k. I would like to add a shunt C on the order of 30-50 nF. Will this cause any stability issues?

0 Carolus Andrews over 3 years ago in reply to David Philpott

TI__Mastermind 18790 points

David,

that amount of resistance should dominate the output impedance such that it should be fine.

Keep in mind that this RC filter is massive. By my calculations, examining the larger values you've presented, the cutoff frequency you've established here is approximately 6Hz, so the device will be sluggish to provide anything more than an averaging response of the input. Is this your intention?

0 David Philpott over 3 years ago in reply to Carolus Andrews

Prodigy 150 points

Carolus,

Thanks for the reply.

I was try to use a simple single stage RC filter (to save board space) to find the avg current of a 6kHz pulsed waveform with ~1mV ripple. I am also using the direct output for an instantaneous measurement. The INA240 was chosen for its space enhanced plastic variant (:

-David

0 Carolus Andrews over 3 years ago in reply to David Philpott

TI__Mastermind 18790 points

David,

Understood. I took the liberty of verifying via TINA-TI, and all looks as expected for a duty cycle of 0.5, 6kHz w/f. As I mentioned above, the time constant here is 25ms, so a 98% t_rise of 4 time constants is around 100ms on startup. Make sure you look at the convergence time you need as duty cycle varies to ensure you know where you are achieving a valid average, but this should be achievable:

INA240 Output Filter.TSC

0 David Philpott over 3 years ago in reply to Carolus Andrews

Prodigy 150 points

Carolus,

I did the same. That response time is adequate for our system. Thanks for your through response.

Have a great afternoon!

-David

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INA240EVM: Maximum capacitive load