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Entry  Wed Feb 20 23:25:50 2013, Zach, Summary, Electronics, Replacement for the AD743: OPA140 and OPA827 ULN_opamp_comparison_2_20_13.png
    Reply  Sat Feb 23 18:01:38 2013, Zach, Summary, Electronics, Replacement for the AD743: OPA140 and OPA827 OPA140_OPA827_noise_vs_Rs.pngScreen_Shot_2013-02-24_at_12.12.23_PM.pngScreen_Shot_2013-02-24_at_12.12.43_PM.png
       Reply  Sun Feb 24 16:49:00 2013, rana, Summary, Electronics, Replacement for the AD743: OPA140 and OPA827 
Message ID: 8125     Entry time: Wed Feb 20 23:25:50 2013     Reply to this: 8151
Author: Zach 
Type: Summary 
Category: Electronics 
Subject: Replacement for the AD743: OPA140 and OPA827 

I have found two great FET input chips that rival the storied, discontinued AD743. In some ways, they are even better. These parts are the OPA140 and the OPA827.

Below is a plot of the input-referred voltage noise of the two op amps with Rsource = 0, along with several others for comparison. The smooth traces are LISO models. The LT1128 and AD797 are BJT-input parts, so their voltage noise is naturally better. However, the performance you see here for the FET parts is the same you would expect for very large source impedances, due to their extremely low current noise by comparison. I have included the BJTs so that you can see what their performance is like in an absolute sense. I have also included a "measured" trace of the LT1128, since in practice their low-frequency noise can be quite higher than the spec (see, for example, Rana's evaluation of the Busby Box). The ADA4627 is another part I was looking into before, the LT1012 is a less-than-great FET chip, and the AD797 a less-than-great BJT.

As you can see, the OPA140 actually outperforms the AD743 at low frequencies, though it is ~2x worse at high frequencies. The OPA827 comes close to the AD743 at high frequencies, but is a bit worse at low ones. Both the OPA140 and OPA827 have the same low-frequency RMS spec, so I was hoping it would be a better all-around part, but, unfortunately, it seems not to be.

The TI chips also have a few more things on the AD743:

  • Input current noise @ 1kHz
    • AD743: 6.9 fA/rtHz
    • OPA827: 2.2 fA/rtHz
    • OPA140: 0.8 fA/rtHz (!)
  • Input bias (offset) current, typ
    • AD743: 30 pA (40 pA) --- only for Vsupply = ±5 V
    • OPA827: ±3 pA (±3 pA) --- up to ±18V
    • OPA140: ±0.5 pA (±0.5 pA) (!) --- up to ±18V
  • Supply
    • Both OPA140 and OPA827 can be fed single supplies up to 36V absolute maximum
    • The OPA140 is a rail-to-rail op amp

These characteristics make both parts exceptionally well suited for very-high source impedance applications, such as very-low-frequency AC-coupling preamplifiers or ultra-low-noise current sources.

 ULN_opamp_comparison_2_20_13.png

(Apologies---the SR785 I was using had some annoying non-stationary peaks coming in. I verified that they did not affect the broadband floor).

R.I.P., AD743

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