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I connected the V- pin to ground and the V+ pin to +5V. I then connected in+ to ground and in- to +5V. I expected Vout to be a low value near ground (0V) but I'm getting about +1.7V. Is this normal?

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    \$\begingroup\$ When running a +/- op-amp on a single supply voltage it is usual to bias the in+ to somewhere in-between ground and V+ (say at 50% with 2 matching resistors in a voltage divider). This provides a 'fake' ground signal. V+ becomes +½V and ground becomes -½V. \$\endgroup\$ – Majenko Jun 18 '11 at 13:56
  • \$\begingroup\$ @Matt, thanks that does make sense. But I'm trying to build a voltage controlled current source and these limitations seem to mean that I'll need a ton of other components to hack it into a workable circuit. Time to order new parts methinks. \$\endgroup\$ – Steven Lu Jun 18 '11 at 14:15
  • \$\begingroup\$ @Steven if you don't mind the delay you could get some free samples from National, Analog Devices, Maxim, etc... \$\endgroup\$ – Majenko Jun 18 '11 at 14:30
  • \$\begingroup\$ @Matt - I think Steven first wants to know what to order (either as sample or purchase) :-) \$\endgroup\$ – stevenvh Jun 18 '11 at 14:37
  • \$\begingroup\$ @stevenvh which is where your answer comes in. These comments are not just for the benefit of the OP but for others who come after. \$\endgroup\$ – Majenko Jun 18 '11 at 14:43
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Standard opamps can't work with input voltages close to the power supply, neither V+ or V-. Same for the output: they don't go all the way to V+ or V-. What you need is a rail-to-rail I/O opamp. (There are also opamps which are only input or output rail-to-rail!).
Also, most opamps want a dual supply, often minimum +5V/-5V. Since you're using it as a single supply opamp, your V+ - V- difference may be too low for normal operation.
Look for a single supply opamp. They often work at low power supply voltages only, and most of them will be rail-to-rail I/O.

note: actually, any dual supply opamp can be used as a single supply; since voltage is relative the opamp doesn't know whether V- is a negative voltage or ground. By single supply opamp I mean specifically an opamp which is low voltage, like I mentioned above.

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  • \$\begingroup\$ Does the LM324DR (which is a quad) fulfill this role? I'd like it to be able to drive Vout down to ground while using it with a single supply (so it will work with PWM). \$\endgroup\$ – Steven Lu Jun 18 '11 at 14:12
  • \$\begingroup\$ @Steven - the LM324 is a classic, a bit like \$\mu\$A741. It works from 3V (good) but isn't rail-to-rail (bad). This overview says "Large output voltage swing 0V to V+ - 1.5V". In the datasheet look for "output voltage swing". \$\endgroup\$ – stevenvh Jun 18 '11 at 14:21
  • \$\begingroup\$ @Steven - you really have to look for rail-to-rail \$\endgroup\$ – stevenvh Jun 18 '11 at 14:22
  • \$\begingroup\$ do you know of one off the top of your head which is a rail-to-rail and a bit of a classic? I'd like to stick with component choices that aren't too exotic but it isn't easy to tell at a site like digikey. Also, "push-pull rail-to-rail"? "differential rail-to-rail"? \$\endgroup\$ – Steven Lu Jun 18 '11 at 14:45
  • \$\begingroup\$ I think "single supply" is op-amp code for "goes to the negative rail but not the positive rail". Fortunately, so many modern systems are battery-powered that RRIO op amps are quite easy to find. \$\endgroup\$ – markrages Jun 18 '11 at 14:46

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