Operations Manual
AD122-96 MKIII
17
Appendix II: AD122-96 MKIII Input Considerations
Achieving 20 or more bits of resolution places a heavy demand on the linearity and noise of analog
signals prior to the A/D conversion.
Linearity:
Linearity problems (typically associated with larger signals) should be avoided to prevent
overall signal distortions. Proper circuit design places non-linear devices inside a negative feedback
path, thus allowing the transfer function to depend on a few linear components (such as resistor ratio).
Closer examination reveals that the effectiveness of negative feedback is limited by the open loop gain
of the amplifier (the available gain prior to the feedback action). Constructing a 10dB gain stage out
of, say, a 50dB open loop amplifier provides 40dB of feedback; thus, circuit nonlinearities are
corrected for by a factor of 100. A 1% nonlinearity reduces to a .01% level. The same stage, when set
to a gain of 20dB, yields only 50-20=30dB of "linearizing power"; thus the same 1% circuit
nonlinearity is corrected to 1/31.6 = .03%. The "obvious" solution of increasing circuit open loop gain
is riddled with problems ranging from impulse response settling time to loss of bandwidth.
Noise:
The quantization (A/D conversion) of analog noise causes loss of fine signal detail. The
quantized noise introduces time varying signal dependent distortions. Such sample by sample
deviation can not be compared to the seemingly "constant" analog background noise. The interaction
between the noise and the quantization process effects the sonic detail at much higher amplitude than
the noise level itself. The source material may have its own noise level, but the A/D manufacturer is in
no position to decide which part of the material is signal and which part is noise. (Is it undesirable
noise or is it the intentional recording of the wind blowing through the trees?) Thus, any signal must be
reproduced accurately. All we can do, and all we should do amounts to avoiding additional A/D noise.
A/D noise consists of two parts: analog noise (prior to quantization), and quantization noise. The
quantization noise level is "fixed" by the converter itself.
Model AD122-96
MKIII
quantization noise
relative full scale is about 126dBFS. Keeping complete system noise level at 126dBFS requires careful
treatment of the analog path. Each circuit component (especially resistors and semiconductors)
generates its own noise. The complex electron motion within the various materials adds up to a
cumulative noise. Intentional amplification of signals yields undesirable amplification of noise. Once
again, each dB of added gain takes away a dB from the available dynamic range.
AD122-96 MKIII Input Range:
You may suspect by now that our aim is to provide you with an A/D
converter and leave the problem of providing a "hot" signal at the hands of microphone and mic-
preamplifier manufacturers. Like many A/D's,
Model AD122-96
MKIII
provides limited gain (0-
12dB). There are 4 methods for analog input signal interconnection:
1. Balanced/ grounded shield
2. Balanced/ floating shield
3. Unbalanced/ grounded shield
4. Unbalanced/ floating shield.
Grounded shield:
analog cable shield is connected pin 1 of the XLR at both ends of the cable driver
end and A/D end.
Floating shield
: analog cable shield is connected pin 1 of the XLR only at the driver end of cable. The
shield is floating at the A/D side (not connected to pin 1).
