2
\$\begingroup\$

I am building a circuit that attenuates the signal and sends it to 3.3V ADC, so I need to limit max voltage to 3.3V. After testing out some circuits for clamping the voltage(various schottky or zener diodes configurations) I found out that they are not really as precise, for example I managed to get 3.5 or 3.7 Volts when applying for example 5 V sine at the input. I suppose for short duration 3.8V is not so bad for ADC, but ultimately it still not a good thing, especially if I leave it like so for a while.

Is there a way to make it more or less precise?
I tried to find some advice on the net and textbooks, but seems like most of the time people don't really care about precise clamping voltage or if they do they promote to use some special input protection ICs (which are good thing, but I just don't have one around at the moment)

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ If you have a circuit which already attenuates, why not just attenuate it to a max of 3.3V. \$\endgroup\$ – efox29 May 15 '16 at 5:29
  • \$\begingroup\$ It is just voltage divider with offset, i.e. it attenuates the signal 10 times. So if I apply 10V sine it will output 1V sine, but if for example I apply 40V it will give 4V sine. I mean attenuator does not intrinsically limit the voltage to pre-defined value. \$\endgroup\$ – ScienceSamovar May 15 '16 at 5:33
  • 1
    \$\begingroup\$ Usually clamping with shottky is fine. Why do you need anything better? What ADC datasheet says about absolute maximum rating? \$\endgroup\$ – Gregory Kornblum May 15 '16 at 5:33
5
\$\begingroup\$

The reason that people don't care about the precise clamping voltage is because in most cases, it isn't important enough to take special precautions. Unless you have some really unusual circuit configuration (not revealed) a Schottky diode to ground and another one to the ADC supply voltage should suffice in 99+% of cases. Most microcontrollers have built-in clamping for signals of moderate impedance.

You may be chasing after problems that don't actually exist.

\$\endgroup\$
  • \$\begingroup\$ So if understand correctly that the external diodes + internal ADC protection in the end would give what I want(3.3 clamping)? Oh, that makes sense. Thanks. \$\endgroup\$ – ScienceSamovar May 15 '16 at 5:35
  • \$\begingroup\$ To emphasize Richard Crowley's point, it's not necessary to clamp the voltage at exactly 3.3V. The inputs of most IC's can take a few hundred mV above the power rail without damage, as long as there is some form of current limiting. \$\endgroup\$ – Dan Laks May 15 '16 at 10:05
1
\$\begingroup\$

Drive the ADC with an op-amp that includes voltage limiting such as the OPA698M. http://www.ti.com.cn/cn/lit/ds/symlink/opa698m.pdf

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.