| Electronics > Projects, Designs, and Technical Stuff |
| For an opamp input, which network protection is preferable? |
| << < (4/4) |
| schmitt trigger:
To all of you, thanks for your valuable suggestions. While I was researching the topic, I found this very interesting e-book from TI. Similar in tone to Analog's fabled "Analogue Dialogue" booklets. :-+ While the better seasoned engineers will know most of the information, most of us will find very helpful tips to improve our circuits. To me it was chapter 28: The unused op amp: what to do? I always grounded the inputs, but that is probably not the best strategy. http://www.ti.com/lit/ml/slyt701/slyt701.pdf |
| David Hess:
--- Quote from: schmitt trigger on April 17, 2019, 06:17:12 pm ---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. --- End quote --- Use low capacitance low leakage diodes like 2N3904 base-emitter (or base-collector for higher voltage) junctions and a lower input series resistance. Bypass the input series resistance with 1000 picofarads to swamp the capacitance of the diodes and amplifier. --- Quote ---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. --- End quote --- LCR meters generally have trouble making this measurement without help. Diode datasheets usually have a capacitance versus voltage graph. Use two diodes in series to halve the capacitance. Higher reverse voltages lower the capacitance. --- Quote ---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. --- End quote --- I do not know about the capacitance but LEDs often do make good low leakage diodes. |
| TimFox:
Diode capacitance at or very near zero can be high. Instead of returning each diode to ground, you can reverse bias it by connecting to another diode that is forward-biased with a small current; when the protection diode is forward-biased, its current adds to the quiescent current of the other diode and the clamp voltage is two diode drops. I have had good luck with c-b junctions of low-noise audio transistors such as 2N2484 (npn) or 2N4250 (pnp). |
| Navigation |
| Message Index |
| Previous page |