Creating capacitance on PCB board for feedback of OPA657?

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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.

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.

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

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.

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

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.

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.

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

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