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Feb 2016: Why you need an analog front end and how to set...

Posted: Thu Feb 04, 2016 1:00 pm
by awirger
Article February 2016, Why you need an analog front end and how to set it up.

I'm trying to build the circuit shown in figure 7 of the article, but I have a few questions.

1. Should the LM356 be driven with a 5.0V supply, or 3.3V.
2. How should the -Rg (pin 1), +Rg (pin 8), and Ref (pin 5) on the AD623 be hooked up? Or are they not connected?
3. Are the +Vs (pin 7) and -Vs (pin 4) on the AD623 hooked up? From the datasheet it looks they supply power. If so, did you use 0 and 3.3V, or 5V.

Thanks in advance.

Update: 2/11/2016
I overlooked a paragraph in the article. It turns out both the Lm356 and AD623 should be powered off 5V.
Across the -Rg and +Rg pins on the AD623, use an 11k resistor to achieve a gain of 10. (Rg = 100k / (G - 1)).
I hooked the Ref pin on the AD623 to ground.
+Vs and -Vs on the AD623 are power. Hook up to 5V.
The two sensitive voltages (for the TMP36 and -IN on the AD623) were driven by the 3.3 reference voltage.

Re: Feb 2016: Why you need an analog front end and how to se

Posted: Sat Feb 06, 2016 8:46 am
by UhClem
It is best if the opamp has a supply voltage no higher than that of the ADC input it is driving. If the opamp was running from a 5V supply while the ADC was running on 3.3V, the ADC inputs could latch up if the opamp output went above its 3.3V supply.

The AD623 is overkill here. It is wildly more expensive than a standard opamp and isn't required.

You can adjust the range and offset of the temperature sensor output using regular opamps. Figure 27 in the TMP36 data sheet shows how.