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| For an opamp input, which network protection is preferable? |
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| schmitt trigger:
Wow!! Excellent advice Tim. I find the idea of bootstrapping the ESD diodes very clever. For the 10k input resistor, I had already considered using multiple devices in series, both to increase power rating and the voltage withstanding capabilities. As I mentioned in my original post, the purpose of this little circuit is to extend the low level range, and increase the input impedance of an analog meter (full scale = 2.5 volts AC, 9kohm/volt). As such I don't require terribly high accuracy (0.5% would be excellent) nor ultra-high impedance (although 1 Meg would be nice to achieve). The meter is reasonably accurate to 100 Khz, so I would also like that bandwidth, too. Initially, I had wanted the amp to also be useful at DC, but if the offset proves to be a chore, I can always AC couple the output. |
| iMo:
Do you need the diodes actually? With 100k input resistor and say 5mA opapm's clamping diodes max current the input voltage could be around 1000Vpp. |
| schmitt trigger:
As I mentioned to Marco, I wasn't seeing the forest because I was focused on a tree. But this discussion so far has been very enlightening, and I've learned a pair of new tricks which I wasn't aware before. |
| Kalvin:
LEDs and diodes will produce some current (read:offset voltage into op amps inputs) when exposed to light, so use a good enclosure which will block light 100%. |
| magic:
--- Quote from: schmitt trigger on April 18, 2019, 12:29:53 am ---I cannot have low voltage offset AND at least 1Meg input impedance AND overload protection AND wide bandwidth. --- End quote --- Perhaps some of TI's e-trim opamps? Picoamps of input current, okay input capacitance for a FET, microvolts of offset and offset thermal drift, reasonable noise, good CMRR up to 3V below VCC or so, rated for ±10mA input overload, a few MHz of GBW, not terribly expensive too. OPA191, OPA192, OPA376 are types I know. |
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