Creating capacitance on PCB board for feedback of OPA657?

I am designing a detector using transimpedance amplifier - OPA657. We are using simple 2 layer PCB board. On TI's datasheet they suggest using 0.1pF capacitor in parallel in 200K feedback resistor.

formatting link
On the previous design we had - 1pF(?!) was used in parallel with ~130K feedback resistor using OPA655.
formatting link

1pF(?I - not sure if it was 1pF as it is impossible to measure it) capacitor was obtained simply using parasitic capacitance of the empty pad. If the empty pad was not there, the gain would be very low (10x lower), so the feedback resistance had to be increased significantly (5x). How do I get ~0.1pF parasitic capacitance for OPA657? And if you know a good guide for calclulating parasitic capacitance of PCB traces

- it would be great as well.

Thanks.

Reply to
m.vitaliy
Loading thread data ...

a écrit dans le message de news: snipped-for-privacy@i40g2000cwc.googlegroups.com...

Use 3 caps in a T connection. This is equivalent to 3 caps, Delta connected. Only one cap matters to your feedback network, the 2 others are absorbed into the output and input parasitics. You can make the central cap adjustable, with the added advantage that it have it's frame grounded.

0.1pF is pretty easy to achieve, provided you've correctly done the layout (not too much parasitics from output to input).
--
Thanks,
Fred.
Reply to
Fred Bartoli

Don't forget the feedback resistor's self capacitance. The through-hole resistors I've measured have been between 0.07 and 0.20pF of self capacitance, with most between 0.10 and 0.15pF. The SMDs I've measured have been slightly higher.

As far as the best desired value, this depends as much on the sensor and node-wiring capacitance as it does on the feedback resistor. Using the appropriate formula works better than an ad-hoc suggestion.

--
 Thanks,
    - Win
Reply to
Winfield Hill

That is a bit odd. My impression was that the parallel resistance of L-trimmed surface mount resistors was appreciably lower than that of spiral=trimmed axial lead through-hole resistors - an impression formed by a Philips video pre-amp on a plumbicon camera tube I ran into on the Cambrdige Instruments Chip-Check mask inspection tool which was based on a Metals Research Quantimet image analysis system.

The original Quantimet system had used a Cambridge Instruments pre-amplifier built with through resistors, and the feedback resistor had been limited to 130k by its paralllel capacitance and the requirement to maintain a video bandwidth. The Philips pre-amplifier that came with the new, improved camera tube, had something closer to

500k of SMD feedback resistance and still gave the same bandwidth.

Nobody had noticed, that this resistor was larger, which messed up all the biassing arrangements, and gave us rotten noise levels - I got slung in as a trouble-shooter, and found and fixed the problem a day or two, which did my reputation no harm at all.

The OPA657 data sheet has a good wirte-up, apparently taken over fron the old OPA655 data sheet, which I've recommedned here before.

formatting link

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

That is a bit odd. My impression was that the parallel resistance of L-trimmed surface mount resistors was appreciably lower than that of spiral=trimmed axial lead through-hole resistors - an impression formed by a Philips video pre-amp on a plumbicon camera tube I ran into on the Cambrdige Instruments Chip-Check mask inspection tool which was based on a Metals Research Quantimet image analysis system.

The original Quantimet system had used a Cambridge Instruments pre-amplifier built with through resistors, and the feedback resistor had been limited to 130k by its paralllel capacitance and the requirement to maintain a video bandwidth. The Philips pre-amplifier that came with the new, improved camera tube, had something closer to

500k of SMD feedback resistance and still gave the same bandwidth.

Nobody had noticed, that this resistor was larger, which messed up all the biassing arrangements, and gave us rotten noise levels - I got slung in as a trouble-shooter, and found and fixed the problem a day or two, which did my reputation no harm at all.

The OPA657 data sheet has a good wirte-up, apparently taken over fron the old OPA655 data sheet, which I've recommedned here before.

formatting link

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

I wasn't too surprised after considering the considerably-closer spacing of a surface-mount part's endcaps, compared to thin-lead 1/4-watt axial-lead resistors, etc. Ohmite's larger SlimMOX HV resistors get down to under 0.04pF, far better than the miniature SMD blokes.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Hi Fred,

I am not quite sure how I would do T connection in this case. Could you please explain? I will be using all SMT componentes, and I haven't even seen the adjustable SMT resistors (do they exist)?

Thanks,

Vitaliy

Reply to
Vitaliy

I have SMDs and was aware of their parasitic effects, never knew their exact values, so thanks for sharing the information.

I will be doing something similar to Figure 3 on OPA657 datasheet, so I will definitely go over that.

Thanks for your help guys. Vitaliy

Reply to
Vitaliy

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.