Cirrus Logic CS4373A - Manual (Page 21)

CS4373A
DS699F2 21
In DC differential output mode (MODE 5) the
level-shifting buffer circuitry adds low-level
32 kHz switched-capacitor noise to the DC
output. This noise is out of the measurement
bandwidth for systems designed with
CS3301A / CS3302A amplifiers and
CS5371A / CS5372A modulators, and is re-
jected by the CS5376A digital filter. This
32 kHz switch-capacitor noise does not affect
DC system tests, though it may be visible on
an oscilloscope at high gain levels.
By measuring both DC test modes
(MODE 4, 5), precision gain-calibration coeffi-
cients can be calculated for the measurement
channel. By first measuring the differential off-
set of the DC common mode output (MODE 4)
and then measuring the DC differential mode
amplitude (MODE 5), a precise offset correct-
ed volts-to-codes conversion ratio can be cal-
culated. This known ratio is then used to
normalize the full-scale amplitude using the
CS5376A digital filter GAIN registers to match
other channels in the measurement network.
By switching between DC common mode
(MODE 4) and DC differential mode
(MODE 5), pulse waveforms can be created to
characterize the step response of the mea-
surement channel. If a pulse test requires pre-
cise timing control, an external controller
should directly toggle the MODE pins of the
CS4373A to avoid delays associated with writ-
ing to the CS5376A digital filter GPIO regis-
ters.
Sensor impedance can be measured using
DC differential mode (MODE 5), provided
matched series resistors are installed between
the BUF analog outputs and the sensor. Ap-
plying the known DC differential voltage to the
resistor-sensor-resistor string permits a ratio-
metric sensor impedance calculation from the
measured voltage drop across the sensor.
Switching between DC differential mode
(MODE 5) and sleep mode (MODE 0, 7) can,
in the case of a moving-coil geophone, test ba-
sic parameters of the electro-mechanical
transfer function. The voltage relaxation char-
acteristic of the sensor when switching the an-
alog outputs from a differential DC voltage to
high impedance depends primarily on the geo-
phone resonant frequency and damping fac-
tor.
OUT+
OUT-
BUF+
BUF-
CS4373A
MODE 4
Approx
-0.15 V
DC
Common
Mode
Approx
-0.15 V
DC
Common
Mode
OUT+
OUT-
BUF+
BUF-
CS4373A
MODE 5
Maximum
2.5 V
DC
Differential
Maximum
2.5 V
DC
Differential
Figure 10. DC Test Modes