OPA858: how to expand

Chi Zhang2

Part Number: OPA858
Other Parts Discussed in Thread: THS4541

Hi Expert,

My customer is using OPA858 in optical module, OPA858 is not rail to rail device, we want to make output swing max, how to optimize the schematic? please give some advice.

Thanks.

Chi

over 2 years ago

0 Sima Jalaleddine over 2 years ago

TI__Mastermind 26455 points

Hello Chi,

Thank you for the schematic; a few questions about the customer's design:

What is the customer's bandwidth requirements?
What is the photodiode's internal capacitance (input cap to the amplifier)?
Input current range of their design
Output voltage requirements of the design
Are they able to add a second stage (no size constraints)
Is their required supply voltage fixed at 5V single supply?

Also, the second image did not go through; please reinsert image.

Thank you,
Sima

0 Chi Zhang2 over 2 years ago in reply to Sima Jalaleddine

TI__Intellectual 2370 points

Hi Sima,

update some comments.

1. bandwidth is 100MHz

2. sorry I couldn't get it

3. 0.22uA-88uA

4.AC swing is 5%-20% DC

5. next stage is THS4541

6. power supply is keeping at 5V

sorry there is no 2nd image, only this schematic.

Chi

0 Sima Jalaleddine over 2 years ago in reply to Chi Zhang2

TI__Mastermind 26455 points

Hello Chi,

Thank you for the additional information!

With a high bandwidth requirement, the OPA858 can handle an input capacitance of around 4 to 5pF at 20kOhms. With the additional gain setting in the feedback, this value would decrease. I would recommend removing the gain switch and using the THS4541 to set the desired gain, or use a differential VGA/PGA (variable or programmable gain amplifier).

A high gain bandwidth amplifier such as the OPA858, will be susceptible to oscillations due to the additional parasitic of the switch in the feedback. Here are two e2e with more information and discussion:

Input current of 0.22uA to 88uA would work with the set voltage bias and looking at the input referred noise figure on the first page of the OPA858 datasheet. Then the rest of the gain for higher output voltage range requirements can be tuned with the THS4541 as mentioned above.

Thank you,
Sima

0 Chi Zhang2 over 2 years ago in reply to Sima Jalaleddine

TI__Intellectual 2370 points

Hi Sima,

thanks for your advice.

we also have some question based on the suggestion.

1. oscillations due to the additional parasitic of the switch in the feedback.

this E2E link has "You can try to add a small phase lead capacitance directly across the -input and output of OPAmp, not only across the gain setting resistors. If adding such a small cap will resuslt in a decrease of noise, you may have cured this instability."

we assume the cap in the path of input and output so make a correction for schematic, could you help to review it? 1st is old one, 2nd is refreshed one.

2. it is better to add RISO between PD and IN- than PD directly connection IN-, my understanding is right?

Chi

0 Sima Jalaleddine over 2 years ago in reply to Chi Zhang2

TI__Mastermind 26455 points

Hello Chi,

I will review the layout and get back to you.
We recommend to place the diode as close to possible to the input of the amplifier. The isolation resistor suggestion is for cases where this is not possible, and a possible cure to instability due to parasitic trace inductance is adding the RISO. There was a case a customer tried this, and there were some stability concerns: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/987143/opa858-another-photodiode-amplifier-for-short-1-2ns-laser-pulse/3659478?tisearch=e2e-sitesearch&keymatch=%20user%3A352398#3659478

Thank you,
Sima

0 kai klaas69 over 2 years ago in reply to Chi Zhang2

Guru 140200 points

Hi Chi,

Chi Zhang2 said:
we assume the cap in the path of input and output so make a correction for schematic, could you help to review it?

It's impossible to help without knowing the detector capacitance and the switch capacitances. So, please provide the detector capacitance and the switch capacitances or tell us what photodetector and what switch you are using.

Kai

0 Chi Zhang2 over 2 years ago in reply to kai klaas69

TI__Intellectual 2370 points

Hi Kai,

the PD cap is 2pF. the Switch is NLAS7222BMUTBG、TS3USB30RSWR、SGM7222YWQ10/TR. please help to check the schematic.

how to understand this " Input current of 0.22uA to 88uA would work with the set voltage bias and looking at the input referred noise figure on the first page of the OPA858 datasheet. " could we take this picture as reference?

Chi

0 Sima Jalaleddine over 2 years ago in reply to Chi Zhang2

TI__Mastermind 26455 points

Hello Chi,

Thank you for sharing the capacitances. That is the correct figure as a reference.

For the above simulation images; these would include the switching capabilities as shared in the first handwritten schematic; is that correct?

Thank you,

Sima

0 kai klaas69 over 2 years ago in reply to Chi Zhang2

Guru 140200 points

Hi Chi,

I'm sorry, I have run several simulations with your circuit but I was not able to achieve the wanted 100MHz bandwidth and to keep the OPAmp stable at the same time.

Also, even the least switch capacitance will considerably decrease the stability of your circuit. So, in any case the chosen USB switches will not work in combination with your circuit.

Kai