I have been using LT1028 in a lot of my projects to amplify bipolar signals before digitization with a (bipolar) analog-to-digital converter. Now I need to use a 5 V unipolar ADC and am trying to figure out level shifting.

The following amplification scheme works according to simulations with LT spice
(red = op amp output, blue = sine voltage input):

enter image description here

but unfortunately not in real life, at least in my attempts. Here is my board layout and some DC voltages that I measured with an oscilloscope.

My question: is the circuit appropriate for the LT1028? If yes, what else could be going wrong? If not, then what is a better circuit to use?

enter image description here

DC potential differences:

  • V+ to AGND = 4.15 V (from Li cell)
  • output to AGND = 3.29 V. In theory, should be ~ 2.0 V
  • VC2 to AGND = 0.128 V

Solution: see accepted answer. Below is the oscilloscope trace showing amplification of a 50 mV, 10 kHz sine wave with AD8061 op amp.

enter image description here

  • \$\begingroup\$ Are you using the actual LT1028 model in your simulation? That's one of the benefits of using Linear parts \$\endgroup\$ – W5VO May 15 '18 at 15:47
  • \$\begingroup\$ No, I am using the ideal op amp \$\endgroup\$ – MichaelT May 15 '18 at 15:49
  • \$\begingroup\$ Thats a really expensive level shifter, I believe the Vcc range is higher than 4V \$\endgroup\$ – Voltage Spike May 15 '18 at 15:52

The LT1028 is incapable of running from a supply of 4 volts. The minimum supply hinted at in the data sheet is +/- 5 volts. You need to use a rail-to-rail op-amp for use in your schematic. Plenty to choose from.

  • \$\begingroup\$ Thanks, I guess I missed that in the datasheet. I'll post back a comment based on findings with a 12 V supply. \$\endgroup\$ – MichaelT May 15 '18 at 15:49
  • \$\begingroup\$ Hold on there - the LT1028 still won't work as you expect. Input common mode range and output voltage swing limitations will scupper that idea - you need to use a rail to rail amplifier or a decent negative and positive supply (min +/- 10 volts for the LT1028). Read the data sheet. \$\endgroup\$ – Andy aka May 15 '18 at 15:52
  • \$\begingroup\$ Can you also suggest other opamps to read about, given that I want to amplify a signal for 0-5 V ADC? I need similar noise performance and gain bandwidth as LT1028. \$\endgroup\$ – MichaelT May 15 '18 at 15:55
  • \$\begingroup\$ No I can't without you telling me who your distibutors are, what country you are in, what budget you desire and what performances you expect from your circuit. Do you see the issue I have? PS I'm not going down that road of course but if you want to do some research and find one I'll be happy to review it. Having said that I have used the ADA4528 in a similar circuit but don't bitch if it's too costly for you. \$\endgroup\$ – Andy aka May 15 '18 at 15:57
  • \$\begingroup\$ Yes sure, I can do the searching. Thanks for the hints. \$\endgroup\$ – MichaelT May 15 '18 at 16:02

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