0
\$\begingroup\$

I have an input sensor with a signal level from 2.3 to 3.5 volts that I want to sample with an ADC. I want to be able to shift this range for the ADC from 0 to 3.5 volts to take full advantage of the ADC dynamic range. I used a summing op amp with an LM741 but it seems to be stuck at 2.3 VDC output all the time.

So the sensor voltage range is Rg = 3.5-2.3 = 1.2

From this Gain = 3.5/1.2 = 2.7

\$\endgroup\$
1
  • \$\begingroup\$ Perhaps including the schematic of the non functioning circuit would help someone explain why a summing amp did not work for you (which, really, should). The kind of output from the sensor or the type of sensor is also relevant information. \$\endgroup\$ Jan 30 '13 at 23:16
2
\$\begingroup\$

Here is a basic circuit that will come close to your requirements.

enter image description here

I showed using a nice opamp from Linear Technology that can work at 5V and still allow the signal to go down near to GND. To get this to work with older, not so nice opamps like a 741, you would need to use supplies of +/-5V or +/-6V.

This shows the response of the above circuit. The output range does not go quite to the limits that you specified for your A/D converter input. It is a good idea to design with some margin to deal with variation in components and the input signal range.

enter image description here

Note that the above circuit inverts the range of the sensor. The inverted opamp configuration was convenient in this case but the circuit could just as well have been designed in a non-inverting manner. On the other hand it takes a trivia amount of MCU software to offset the A/D readings to remove the inversion if that proves necessary.

\$\endgroup\$
2
\$\begingroup\$

The LM741 is not a great option for your requirements. It would require a dual supply with a minimum voltage of +/- 4.5 Volts (or an isolated 9 Volt supply and a virtual ground using a decoupled voltage divider) , has a fairly low input impedance compared to modern op-amps, and far-from-ideal characteristics including offset drift and poor operation close to supply rail voltages.

Consider instead any of the hundreds or thousands of op-amps designed for single-supply low voltage operation with rail-to-rail I/O (RRIO). A parametric search on one of the major manufacturer sites, such as Texas Instruments, will provide many options.

The summing amplifier design should work perfectly for your purposes if the voltage ranges, desired frequency / bandwidth, and supply requirements are kept in mind.


EDIT: Possible alternative op-amp for stated purpose:

If SMD components are acceptable, the Texas Instruments OPA341 and its dual-channel counterpart, OPA2341 are my go-to choices for applications like this.

These op-amps are specified for single supply operation from 2.7 to 5.5 Volts, and outputs can be driven to within 5 mV of each supply rail for a 100 kOhm load - 200 mV each way if load is low impedance, ~2 kOhm.

Texas Instruments offers free samples of both parts on their web site. These are available only in various small SMD packages, however, not in DIP.

For through-hole DIP, my parts shelves contain the Analog Devices AD8044 quad op-amp, which is specified for 3 to 12 Volt single supply, with a whopping 150 MHz GBW. This op-amp can be driven to within 200 mV of each supply rail, and is designed for loads as low as 150 Ohms. Single (AD8041) and dual (AD8042) versions also exist.

Analog Devices offers free samples for these parts on their web site, but like many Analog Devices components, they are a bit expensive if going into production.

For a more specific recommendation, the following parameters would need to be known at a minimum:

  • Signal voltage
  • Signal frequency / bandwidth
  • Vcc of the circuit
  • Whether SMD is acceptable or through-hole DIP parts are required
  • Impedance of source
  • Impedance of load
  • How many op-amp channels are desired per device
\$\endgroup\$
2
  • \$\begingroup\$ Thank you both for the feed back. I see that the 741 is obviously the wrong choice. I used this because I had a bunch left over form an old project. Can you recommend one for a single supply design at 3.3 - 3.5 volts? \$\endgroup\$
    – user18472
    Feb 1 '13 at 13:52
  • \$\begingroup\$ @user18472 Parts that I have in my shelves and would use for purposes like yours, are now added to my answer. However, additional information as noted there may help provide more optimal suggestions. \$\endgroup\$ Feb 1 '13 at 14:27

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.