OPA656: Transimpedance amplifier

Hi Archie,

a schematic would be helpful.

Kai

Hello Archie,

Please share a schematic and/or simulation results so we can best assist you.

Best,

Alec

Well archie, there is both an input offset and Ib shift with temperature (not in sim model), without schematic hard to say, I would note two things, 

1. there is a B grade with screened lower offset drift, 

2. The Decomp OPA657 will give wider BW and a little lower noise if need be. 

Hi Experts,

Thanks for waiting as I have just got the information needed.

See attached. We are certainly under a great deal of pressure to get this product out. We would also appreciate a package equivalent of the op amp if we cannot sort this out. We initially picked the high speed amplifier to ensure the detection of short pulses. However, we may be able to lower the bandwidth of the amplifier but ideally have low bias current.

Thank you.

Regards,
Archie A.

Additional updates:

I have breadboarded the design with a single amplifier and measured the current consumption @ 10 mA per rail. NOTE: This is with the input on. There is no heat on the device at all. Hence my confusion as to why our production sensor board heats up. Also, if required, we can utilize a slower amplifier but the physical footprint MUST be the same. Any thoughts would be appreciated.

Regards,
Archie A.

Hi Archie,

hhm, I'm a bit confused. The amplifier you have shown is no transimpedance amplifier (TIA) but a non-inverting amplifier?

Kai

Hello Kai,

Thanks for supporting.

Yes, apologize. We are using the amplifier in a non-inverting configuration, as we utilize a resistor to convert the sensor current to voltage. We are following the manufacture's approach for this type of sensor.

We selected this amplifier because of the low bias input current, high bandwidth with high gain(150).

Thank you.

Regards,
Archie A.

Hello Archie,

Thank you for clarifying the circuit behavior; I will investigate and check with my team on possibly causes for your heat on the production board.

Could you share any information about the production board, whether it be board files, images, description of the thermal observations, etc?

Best,

Alec

Hi Archie,

the circuit is instable:

archie_opa657.TSC

The reason for instability is that the OPA657 is an OPAmp which needs a minimum gain of 7V/V. But C2 (in my simulation) heavily decreases the gain at high frequencies and destroys the phase margin.

Removing C2 restores the phase margin again and guarantees stability:

Kai

Hi Archie,

also keep in mind that changing from a TIA to a non-inverting amplifier has some disadvantages. You loose all the advantages of the unique TIA topology. This primarily means that the detector capacitance, stray capacitance and the input capacitance of OPAmp will form an unwanted input voltage divider. Also, the photodiode current now causes an unwanted voltage drop across the 27k input resistance and by this changes the bias voltage of photodiode. You don't have any longer a zero bias voltage at the photodiode but a bias voltage which depends on the photodiode current. This means a change of sensitiviy of photodiode depending on the photodiode current.

Kai

Hello Kai/Aec,

Thanks for sharing your insights.

Upon removing the capacitors as you indicated, the opamp still running but in extremely hot (measured at 60 degrees C).

Changing opamp to LMH6609, while the power consumption and heat are way down, this amplifier has a huge output offset with NO signal coming in. Both amplifiers should work with a gain of 150.

Hoping to hear experts response.

Thank you.

Regards,
Archie A.

Hi Archie,

ArTzy said:

Upon removing the capacitors as you indicated, the opamp still running but in extremely hot (measured at 60 degrees C).

When running the OPA657 as a non-inverting amplifier the OPAmp expects to see a low source impedance. But in your circuit it sees a 27k resistor in parallel with a very small detector capacitance. This can destabilize the OPA657. Keep in mind that the OPA657 is a decompensated 1.6GHz OPAmp. Such a fast racehorse cannot be treated like a slow standard OPAmp. It might not work properly in conditions it has not been designed for.

And unwanted load capacitance at the output coming from a non-isolated scope probe can also cause stability issues. Never touch any pin of a HF-OPAmp directly with the scope probe. Insert a suited isolation resistor first. See figure 17 of datasheet. And mount this isolation resistor directly at the pin of OPAmp.

Another cause of instability is the setup. Even only a few millimeters of copper traces at the wrong point can destabilize the OPAmp. Unsuited decoupling caps can destabilize the OPAmp. And omitting a solid ground plane can destabilize the OPAmp.

ArTzy said:

Changing opamp to LMH6609, while the power consumption and heat are way down, this amplifier has a huge output offset with NO signal coming in.

Yes, of course, because the LMH6609 has a typical input bias current of up to 2µA. 2µA times 27k gives a self-inflicted input offset voltage of 54mV, times a gain of 150V/V results in an output offset voltage of 8.1V!

Do you see now that the 27k resistor at the input of a HF-OPAmp is no good idea?

Kai

Hi Expert,

Good day.

Here is the layout. All amplifiers are routed the same way. All components are identified. NOTE: The 4.7pF cap has been removed as per your previous recommendations. No oscillations, perfect performance, just too much heat.

Have a solid ground plane underneath these amplifiers. The loading is quite light. All amplifier components are right beside the pins that utilize said components. So assuming the layout have sorted out. All is working good except for too much heat.

Hope you help this out.

Thank you.

Regards,
Archie A.

Hi Archie,

I think you can make the copper tracks wider, as shown in this layout example:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/924141/opa380-layout-for-signal-inputs-for-opa380

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1033505/opa847-in-this-tia-circuit-can-i-use-series-resistors-dividing-voltages-to-couple--vbias-to-the-pin-s-anode-when-the-dc-voltage-source-is--5v-and-the-anode-voltage-needs-to-be-1v

This will enhance the heat dissipation.

Oops, are you using through hole components? Even for the supply voltage decoupling caps? Not good... way too much inductance.

Kai

Hi Alec/Kai,

The ground plane is solid, and the power supply traces are about .025 wide. All signal traces are between .007 and .013. Could you elaborate what particular part from layout need to adjust? Sorry if the this query has overwhelmed follow ups.

Also, all components are surface mount with said components very close to the input pins. We have seen no oscillations in the lab, so I am not sure why these amps run so hot.

Thank you..

Regards,
Archie A.