Sensor input mode measurements

[Mitch Bradley]

I measured the DC transfer function of the audio system in sensor input 
mode (DC coupled) on a pre-B2 board.  Here are the results:

Executive summary:  At 0dB gain, the DC input range is from 0.41 V to 
1.96 V, with the midpoint (digital 0) at 1.18 V.  The midpoint is 
independent of the digital gain setting.


Test conditions:

MIC input.  Record gain register (index 0x1c) set to 0 (== 0dB gain), 
mic boost off (no +20dB extra gain).  Codec hipass filter off (0x1000 
bit set in register 0x5c). DC coupled input (5536 GPIO01 high).  Vrefout 
(MIC bias) on (2.3 V open circuit). I control the input voltage with a 
26K POT from MICin to ground, measuring the voltage with 3-digit VOM and 
adjusting POT for various voltages.  (The POT forms a voltage divider 
with the 3K MIC BIAS resistor).  Using the OFW audio driver to capture 
1/2 second of samples at 8000 samples/sec, calculating the average value 
of the left channel samples.

The digital output value is 0 at 1.18 V, -32768 at 0.41 V, +32768 at 
1.96 V. Over most of that range, the analog-to-digital gain is 43000 
counts/volt, and appears to be fairly linear within my measurement 
precision.

If I change the record gain register value to 0x808 (+12 dB, a voltage 
gain of 4), the 0 point remains at 1.18 V, and the range shrinks to 0.99 
V .. 1.37 V, which is 1/4 the previous range.

A similar result hold for the maximum record gain setting - the range 
shrinks by the amount corresponding to the voltage gain equivalent of 
22.5 dB.

Note that this input range (1.55 Vpp) is rather less than the AD1888 
data sheet implies (2.83 Vpp), but that might be related to the fact 
that we are running with
Avdd = 3.3 V instead of the 5 V that the data sheet calls out.

Mitch