OPA858: OPA858 output abnormal issue

Hi kai, Alec,

Really appreciate for your reply.  Well noted.

Best regards,

Jasper

Hi Kai, Alec,

I have a new problem. The case is that:

The feedback resistor is 470ohm,  and the GND resistor is 100ohm, so I think the gain is 1+470/100=5.7, when the input is 0.295V, the output should be 1.68V, but the test the output is 2.35V, could you help analyze the reason?

So could you help confirm whether OPA858 is suitable for the same direction scaling application? 

Thanks.

Regards,

Jasper

Hi Jasper,

the OPA858 needs a minimum gain of factor 7. But in your circuit the gain is only 5.7. So the OPA858 might oscillate.

Try to make R89 > 50R. This will isolate the output of OPAmp from capacitive loads and can restore the phase margin a bit.

But why are you using a decompensated 5.5GHz OPAmp? Only for amplifying DC signals?

Kai

Hi Kai,

I will let my customer to adjust the gain to about 10 and will update the result. They just use DC signal in test and the OpAmp in their product is used for amplifying AC signals. Thanks. 

Best regards,

Jasper

Hi Jasper,

what load is connected to the output of OPA858? A critical load with too much capacitance can make stability issues.

Kai

Hello Jasper,

Kai is correct; if you could share more details regarding your load and circuit, we can assist in evaluating your circuit stability, as well as any additional items.

Best,

Alec

Hi Kai, Alec,

I need to check it with my customer about the load and circuit. I will update it when I get it. Thanks for your support.

Best regards,

Jasper

Hello Jasper,

I will await your update.

Best,

Alec

Hi Alec,

I am checking with my customer. Thanks.

Regards,

Jasper

Hello Jasper,

Were you able to have a conversation with your customer?  Has your issue been resolved or addressed?

Best,

Alec

Hi Alec,

My customer feedback that OPA858 is not a rail-to-rail device, so they do not adopt same direction amplification. And they haven't provide their schematic and load circuit. 

Thanks. 

Best regards,

Jasper

Hi Jasper,

As I understand your response, the customer is looking for a device which can perform rail-to-rail output in a gain of 10 V/V for AC signal inputs.  Could you specify the frequency range the customer requires for operation?  What are the characteristics of the AC signal on the amplifier input?

Depending on your customer's load, bandwidth needs, input/output requirements, and schematic, I would be able to continue assisting you in finding a TI op-amp which fits the application.

If you are able to share more information regarding your customer's needs, I can return to this forum post to assist you at a later date.  Until I have more information, I am unable to provide any more advice on selecting an amplifier or troubleshooting performance.

Best,

Alec

Hi Alec,

Thanks. My customer has selected OPA858, and they have finished the schematic. I haven't get clear about their specific requirement as you said. It may take more time but I will update it when I got it. Thanks for your patience.

Best regards,

Jasper

Hello Jasper,

I will wait for your update.  

Best,

Alec

Hi Alec,

Thanks and I will update more information to you later.

Regards,

Jasper