Cirrus Logic CS5373A - Manual (Page 25)

CS5373A
DS703F2 25
calibration and differential pulse tests. In mode
4, both sets of analog outputs (OUT and BUF)
are enabled.
5.3.2 DC Differential
The second DC test mode (MODE 5) enables
the modulator and DC test circuitry to create a
precision differential DC analog output voltage
as the final measurement for gain calibration
and as the step/pulse output for differential
pulse tests. In mode 5, both sets of analog out-
puts (OUT and BUF) are enabled.
In DC differential 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 band-
width for systems designed with a
CS3301A/02A amplifier and CS5373A modu-
lator and is rejected by the CS5378 digital fil-
ter. This 32 kHz switched-capacitor noise
does not affect DC system tests, though it may
be visible on an oscilloscope at high gain lev-
els.
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 along
with the CS5378 digital filter GAIN register to
normalize the full-scale amplitude 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
CS5373A to avoid delays associated with writ-
ing to the CS5378 digital filter GPIO register.
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 modulator mode (MODE 0) can,
in the case of a moving-coil geophone, test basic
parameters of the electro-mechanical transfer
function. The voltage relaxation characteristic of
the sensor when switching the analog outputs
from a differential DC voltage to high impedance
depends primarily on the geophone resonant
frequency and damping factor.
5.4 Sleep Mode
Sleep mode (MODE 7) saves system power
when measurements are not required by turning
off the modulator, AC test circuitry, and DC test
circuitry. In sleep mode the modulator digital out-
puts and the BUF and OUT analog outputs are
high impedance.
OUT+
OUT-
BUF+
BUF-
CS5373A
MODE 4
Approx
-0.15 V
DC
Common
Mode
Approx
-0.15 V
DC
Common
Mode
OUT+
OUT-
BUF+
BUF-
CS5373A
MODE 5
Maximum
2.5 V
DC
Differential
Maximum
2.5 V
DC
Differential
Figure 12. DC Test Modes