Electronics > Projects, Designs, and Technical Stuff
For an opamp input, which network protection is preferable?
schmitt trigger:
I am in the process of designing a 10X preamplifier to extend the range of an analog meter.
The input will be +/- 300mV DC or AC pk-pk. Hopefully I can get a 100 Khz bandwidth.
However, I know that sh!t happens, and require to protect the input against mild overvoltages.
The standard protection network as shown in (A), series schottkys to V+ and V-, works fine. But schottkys have significant leakage, and because of the 1 Meg impedance, it could generate offset voltage errors.
Therefore I was thinking along the lines of inverse parallel diodes from input to ground, as shown in (B). But while the reverse-bias capacitance is well specified, the capacitance below the forward bias is not. Using my Peak Atlas LCR meter, a pair of antiparallel 1N4148 show about 50 pF, which could cause loading at higher frequencies.
I don't know how much I can trust this reading.
I had read somewhere that LEDs, covered to avoid photoelectric effects, show both ultra-low leakage and minimum capacitance. Again using the Peak Atlas, 3 mm green LEDs measures 15 pF.
So what do you think? What would be the best method?
floobydust:
I would just use low-leakage silicon diodes or diode-connected transistors, and assuming that clamp voltage is now down to say +/-1.3V, then the next resistor to limit op-amp input current.
OPA1611 limit is +/-0.5V past the rails and +/-10mA. Unless power could be off when input or ESD is present, so 2V/10mA is around 200R minimum.
I think your impedances are really high 90k, 505k I would use 1/10 of that or lower for lower noise and less susceptibility to moisture.
If I need higher clamp voltages I've used BAV99S quadruple package, for clamps to +/-2.5V at 150mA.
TVS diodes most have high capacitance, although some Ethernet/USB parts are also very good.
Marco:
--- Quote from: schmitt trigger on April 17, 2019, 06:17:12 pm ---However, I know that sh!t happens, and require to protect the input against mild overvoltages.
--- End quote ---
10 mA * 100 kOhm = 1 kV ... forget about it. Almost all active components have protection build in, so that level of series resistance is protection enough.
schmitt trigger:
Thanks Marco!
Now that you mention, it is perfectly obvious.
A case of not seeing the forest because the sight is fixed on the trees.
magic:
OPA1611? There will be up to a few mV of offset error from input bias currents and the rather high and unequal impedances of input networks.
Navigation
[0] Message Index
[#] Next page
Go to full version