INA333: Output oscillating at around 1Hz in my circuit

Andrew Saldanha

Part Number: INA333

Hi,

I am using the circuit shown below. The problem that I have is it is unstable. With no input signal it begins to oscillate at the output of the IA at around 1Hz, Do you have any idea why it would do this?

over 5 years ago

0 Dennis Goeke over 5 years ago

TI__Genius 11135 points

Andrew

The schematic did not come through. Please try again. Use the link at the lower right "Insert Code, Attach Files and more..."

Regards
Dennis

0 Andrew Saldanha over 5 years ago in reply to Dennis Goeke

Prodigy 140 points

INA333 circuit.docxHi Dennis,

I have attached the schematic. Hope that worked.

0 kai klaas69 over 5 years ago in reply to Andrew Saldanha

Guru 140200 points

Hi Andrew,

with such a high gain the +2.5V reference voltage must be very well regulated, stable and clean. Any source impedance of this +2.5V reference voltage will be critical. How do you generate the +2.5V reference voltage?

Kai

0 Andrew Saldanha over 5 years ago in reply to kai klaas69

Prodigy 140 points

kai klaas69 said:
Hi Andrew,

with such a high gain the +2.5V reference voltage must be very well regulated, stable and clean. Any source impedance of this +2.5V reference voltage will be critical. How do you generate the +2.5V reference voltage?

Kai

kai klaas69 said:
Hi Andrew,

with such a high gain the +2.5V reference voltage must be very well regulated, stable and clean. Any source impedance of this +2.5V reference voltage will be critical. How do you generate the +2.5V reference voltage?

Kai

Hi Kai,

see the attached doc. I use a voltage divider and buffer the reference voltage.

Reference voltage.docx

0 kai klaas69 over 5 years ago in reply to Andrew Saldanha

Guru 140200 points

Hi Andrew,

the ADA4505 is a 10µA supply current OPAmp and its short circuit current is only 3.8mA. Hmm, for my taste the ADA4505 is too weak and cannot properly work as a buffer.

I would try to spend some of the +2.5V nodes an own ADA4505 and by this decouple the 2.5V nodes from each other.

Kai

0 Andrew Saldanha over 5 years ago in reply to kai klaas69

Prodigy 140 points

Thanks for that observation Kai. I did overlook the drive current.

0 Andrew Saldanha over 5 years ago in reply to Andrew Saldanha

Prodigy 140 points

I had another look at the data sheet. Figure 18 of the data sheet shows the load current and it seems that the load current is sufficient for what I need. I think the 10uA current spec is the per channel power supply current under no load.

0 Thomas Kuehl over 5 years ago

TI__Guru* 93105 points

Hi Andrew,

I don't know if you have this INA333 issue resolved yet so I am checking to see what progress has been made.

If the 1 Hz oscillation is still occurring can you capture an image of the output waveform and post it? I do suggest checking all the other INA333 pins with a DSO and see if there is any evidence of the oscillation on any of those pins.

What potential is Tpad_1 connected to in your circuit?

Regards, Thomas
Precision Amplifiers Applications Engineering

0 Andrew Saldanha over 5 years ago in reply to Thomas Kuehl

Prodigy 140 points

Hi Thomas,

I have resolved the issue by lowering the gain of the INA333 and following OpAmp, so the total gain is 2000. I have shown some 5Meg feedback resistors which are part of the circuit in the attachment. I don't know if they could be the cause or if the circuit is just sensitive at high gain. Those feedback resistors are part of another chip that is coupled to this circuit.

The Tpad is connected to ground.

I will check the other pins if I see the oscillation again. Thanks for the tip

0 Andrew Saldanha over 5 years ago in reply to Thomas Kuehl

Prodigy 140 points

Hi Thomas,

correction to my previous statement, TPad is not connected to any potential.

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INA333: Output oscillating at around 1Hz in my circuit