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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: 8151     Entry time: Sat Feb 23 18:01:38 2013     In reply to: 8125     Reply to this: 8153
Author: Zach 
Type: Summary 
Category: Electronics 
Subject: Replacement for the AD743: OPA140 and OPA827 

Rana suggested that I measure the OPA827 and OPA140 noise with high source impedance so as to see if we could find the low-frequency current noise corner. Below is a plot of both parts with Rs = 0, 10k, and 100k.

As you can see, both parts are thermal noise limited down to 0.1 Hz for up to Rs = 100k or greater. Given that the broadband current noise level for each part is ~0.5-1 fA/rtHz, this puts an upper limit to the 1/f corner of <100 Hz. This is where the AD743 corner is, so that sounds reasonable. Perhaps I will check with even higher impedance to see if I can find it. I am not sure yet what to make of the ~10-20 kHz instability with high source impedance.

OPA140_OPA827_noise_vs_Rs.png

EDIT: The datasheets claim that they are Johnson noise limited up to 1 Mohm, but this is only for the broadband floor, I'd guess, so it doesn't really say anything about the low frequency corner.

Screen_Shot_2013-02-24_at_12.12.23_PM.png Screen_Shot_2013-02-24_at_12.12.43_PM.png

Quote:

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.

 

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