AD633 - trim or not?

[Toby Paddock]

Warning- This got kind of long and rambling.

I thought I'd do a quick check on my AD633 ring-mod 
breadboard to see the difference between trimmed and 
grounded. But what I found was a dependence
on the impedance hung on the input.

Breadboard and such:
X INPUT to the board has 100k to ground and a 
series 100k to the X+ of the chip. X- is trim.
Y is the same.
Output is the output pin of the chip. No gain.
All DC coupled.
Power is +/-15V. Pretty good ground plane, 
but all the leads are kind of long.
Trimmer voltages are from +/- 0.7V 
resistors/diodes ref voltage.
Input signal is 5V 1kHz sine (+/- 2.5V).
All AC voltages are pk-pk.
In all modes, there is about 7mV 
'high freq' noise on the output.
INPUT means X and Y inputs at the board edge.

Trimmed mode:
Signal to X INPUT. Grounded Y INPUT (Note 1). Trimmed Y- 
for min bleedthru. Measured 1-2 mV out and frequency is 
doubled. (Like ripple of a full wave bridge)

Signal to Y INPUT. Grounded X INPUT (Note 1). Trimmed X- 
for min bleedthru. Couldn't see any signal in the noise.

Untrimmed mode:
X- and Y- are directly grounded.
Signal to X INPUT. Grounded Y INPUT (Note 1). Measured 14 mV out. 
Signal to Y INPUT. Grounded X INPUT (Note 1). Measured 15 mV out. 
(Note 2)

But then I noticed:
When I removed the ground from an INPUT, I had to re-trim.
So I guess that when a chip input pin has series 
resistance, the bias current (or whatever it's called) out of the 
pin develops a voltage offset. So then you need to re-trim 
depending on the impedance of what's attached to the INPUT.
I do remember that when I was using it as a ring-mod I 
would have to re-trim depending on what I connected to it. 
(A Fisher-Price Sing-Along must be AC coupled)

So:
Do I need to add buffers to the INPUTs? Icky, another chip.
I guess AC coupling the INPUTS when used for audio would help.

Note 1: This is at the breadboard input, so the chip sees 100k 
to ground. Didn't know that it mattered at the time. When INPUT 
is grounded the chip sees 100K to ground. When open, 
it sees 200k to ground.

Note 2: What would happen if the X- and Y- were grounded 
thru the same value of R that the X+ and Y+ see?

Advice, questions, corrections, and personal abuse 
are more than welcome.

 - -- -  Toby Paddock