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OPA1642: OPA827, OPA2140 & OPA1642 source impedance and distortion issue.

Petr Novak51
Prodigy 20 points
Part Number: OPA1642
Other Parts Discussed in Thread: OPA2140, OPA827, , OPA140

OPA827, OPA2140 & OPA1642 source impedance and distortion issue.
 
Dear Sir/Madam,
 
While reviewing the datasheet OPA1642, by chance, I learnt the specifications in chapter 8.1.4 Source Impedance and Distortion had changed significantly; please find attached both versions of the datasheets (OPA1642old and OPA1642new).
 
In 2013, I bought larger quantities of OPA1642 and OPA2140 for one of my clients. Would you kindly advise how I can discern the new version of the operational amplifier OPA1642 from the old one? It seems the code OPA1642 is the same in both.
 
In addition, would you let me know whether the same technological change had been implemented in the high precision version operational amplifier OPA2140? If so, how I can recognise the old and the new version?
 
I bought the operational amplifiers from a wholesaler who may have a stock of old versions of OPA1642, OPA2140, OPA827 selling them even today, hence, I need to know how to recognise the versions so that I may adapt the circuits accordingly.
 
Quite often, in my circuits, I need to use operational amplifiers with JFET-Inputs in noninverting configurations and variable input impedance in which I cannot achieve input impedance matching for maintaining low distortion, e.g. in ECG input amplifiers with high and variable input impedance (human body variable resistance).
 
I use the operational amplifier OPA827 most of the time, however, the datasheet does not mention input impedance matching for maintaining low distortion; hence, my question is if this circuit in voltage follower configuration requires input impedance matching to maintain low distortion.
 
At present, I need to use operational amplifiers OPA1642 & OPA2140 in the modified circuit on Figure 1. (IC403, IC404) as a voltage follower with high input impedance.
No coupling capacitors are used in the modified circuit except C417, C418, C423, C424, so, it would be better to use the operational amplifier OPA2140 instead of IC404 high-precision version. As an alternative, I am using operational amplifiers LT1469 with low distortion and very good DC parameters instead of IC403, IC404, however, from the noise figure perspective, it would be better to use JFET-Input operational amplifiers with good DC parameters.
 
Thank you in advance for your time and guidance.
 
Yours respectfully,
 
Petr Novak