Jung, Self and others have written about the use of impedance balancing resistors to minimize non-inverting op amp common mode distortion. Self writes in Small Signal Design:
A voltage-follower has no inconvenient medium-impedance feedback network, but it does have a much larger CM voltage. Figure 4.10 shows a voltage-follower working at 5 Vrms. With no source resistance the distortion is quite low, due to the 100% NFB, but as soon as a 10 kΩ source resistance is added we are looking at 0.015% at 10 kHz.
Once again, this can be cured by inserting an equal resistance in the feedback path of the voltage-follower, as in Figure 4.3d above. This gives the ‘Cancel’ trace in Figure 4.10. Adding resistances for distortion cancellation in this way has the obvious disadvantage that they introduce extra Johnson noise into the circuit. Another point is that stages of this kind are often driven from pot wipers, so the source impedance is variable, ranging between zero and one-quarter of the pot track resistance. Setting a balancing impedance in the other opamp input to a mid-value, i.e. one-eighth of the track resistance, should reduce the average amount of input distortion, but it is inevitably a compromise.
This post explores the common mode distortion of a voltage follower with various degrees of impedance imbalance in series with the inverting (feedback) and non-inverting inputs.
Resistors are also often inserted in series with one of the inputs to protect the back-to-back diodes across many op amp's differential inputs. The NE5532 and LME49720 both have differential protection diodes. This is JRC's recommendation:
For protection purposes I'm not sure it makes much difference if the 1K is inserted in series with the inverting input instead of the non inverting one.
The test circuit uses an ULDO-Nacho oscillator/filter to feed the DUT. The output of the DUT feeds the 1 kHz notch filter input which has +40 dB of post-filter gain.
The tests are at +20 dBu (just below the clipping point with +/-15V supplies) and at +14 dBu which is the output level of an attenuator fed by +20 at it's maximum Thevenin source impedance point at -6 dB attenuation.
The above figure is simplified. Ignore for the sake of this discussion op amp bias current in the wiper.
A 10KΩ pot has its maximum source impedance of 2.5 KΩ at mid-rotation.
In the +14 dBu tests the most relevant ones for my purposes (buffering a 10K stepped switch) are those made with a 2K49Ω source impedance.
A 1M pt FFT is performed on the Nacho notch filter output. The y-axis displays dBc (dB below carrier).
The first FFT is the oscillator loopback. THD calculation requires correction for the notch filter insertion. The actual THD is displayed below the FFT.
Oscillator Loopback at +20 dBu
THD 0.0000072%; 0.07 ppm; -142 dBc
5532 Rs=0Ω Rfb=0Ω at +20 dBu
THD 0.00003297%; 0.3297 ppm;-129.64 dBc
5532 Rs=0Ω Rfb=1KΩ at +20 dBu
THD 0.00004597%; 0.4597 ppm; -126.75 dBc
Note that although the third harmonic has increased significantly in the 0/1KΩ example compared to 0/0Ω the overall distortion signature is simpler with less higher-order components. The second harmonic is at a similar level to the 0/0Ω measurements and thus, due to it's increased weighting from the filter's 9 dB insertion loss, dominates the overall THD measurement. (IMO third harmonic compressive distortion, which intermodulates less than second, is less audible. To the ear even-order also carries greater weight.)
5532 Rs=499Ω Rfb=499Ω at +20 dBu
THD 0.00002628%; 0.2628 ppm; -131.61 dBc
5532 Rs=2K49Ω Rfb=0Ω at +20 dBu
THD 0.00006644%; 0.6644 ppm; -123.55 dBc
5532 Rs=2K49Ω Rfb=1KΩ at +20 dBu
0.00004735%: 0.4735 ppm;-126.49 dBc
5532 Rs=2K49Ω Rfb=2K49Ω at +20 dBu
0.00002917%; 0.2917 ppm; -130.70 dBc
Some measurements at +14 dBu
Oscillator Loopback at +14 dBu
THD 0.00000980%; 0.0980 ppm; -140.18 dBc
5532 Rs=2K49Ω Rfb=0Ω at +14 dBu
THD 0.00002113%; 0.2113; -133.50 dBc
5532 Rs=2K49Ω Rfb=1KΩ at +14 dBu
THD 0.00001804%; 0.1804 ppm; -134.88 dBc
My conclusions for the NE5532 are that if current-limiting resistors are used to protect the diodes the CM distortion is lower if the 1K is split into two 499Ω in series with each input. 499/499Ω has about 2 dB less distortion than 0/0Ω.
This assumes that the driving source impedance is a "0Ω" op amp output. The resistors cannot be made too large or they'll introduce Johnson noise.
If the follower is going to be used to buffer a 10KΩ pot or stepped attenuator the 1K Rfb seems to be a good compromise between full on and - 6dB attenuation. The 2K49/1KΩ configuration has a 1.4 dB distortion advantage over the 2K49/0Ω values at +14 dBu.
With a 1K Rfb the THD varies from -126.75 dBc at +20 dBu (attenuator full on 0/1kΩ) to -134.88 dBc at 6 dB attenuation. (2k49/1KΩ)
Compared to a 0Ω Rfb example +20 dBu distortion is degraded by about 3 dB (mostly third) but at +14 dBu its about 1.4 dB better.
Making Rfb 0Ω is not really an option however because it eliminates input diode protection.
It has also been shown that a 0Ω Rfb produces a more complex higher-order distortion signature.
I'll post some quick OPA2134 measurements which show, in this configuration, the superiority of the 5532.