Crystal CS5521-AP, General Description Manual

Page: 22 / 38

CS5521 CS5523
22 DS317PP2
to AGND. The choice of the operating mode for the
NBV voltage depends upon the input signal and its
common mode voltage.
For the 25 mV, 55 mV, and 100 mV input ranges, the
input signals to AIN+ and AIN- are amplified by the
20X instrumentation amplifier. For ground refer-
enced signals with magnitudes less then 100 mV, the
NBV pin should be biased with -1.8 V to -2.5 V. If
NBV is tied between -1.8 V and -2.5 V, the (Com-
mon Mode + Signal) input on AIN+ and AIN- must
stay between -0.150 V and 0.950 V to ensure prop-
er operation. Alternatively, NBV can be tied to
AGND, where the input (Common Mode + Signal)
on AIN+ and AIN- must stay between 1.85 V and
2.65 V to ensure that the amplifier operates proper-
ly.
For the 1 V, 2.5 V, and 5 V input ranges, the instru-
mentation amplifier is bypassed and the input sig-
nals are connected to the Programmable Gain
block. Whether NBV tied between -1.8 V and
-2.5 V or tied to AGND, the (Common Mode +
Signal) input on AIN+ and AIN- must stay between
NBV and VA+.
The CS5521/23 can accommodate full scale ranges
other than 25 mV, 55 mV, 100 mV, 1 V, 2.5 V and
5 V by performing a system calibration within the
limits specified. See the Calibration section for
more details. Another way to change the full scale
range is to increase or to decrease the voltage refer-
ence to other than 2.5 V. See the Voltage Refer-
ence section for more details.
Three factors set the operating limits for the input
span. They include: instrumentation amplifier satu-
ration, modulator 1’s density, and a lower reference
voltage. When the 25 mV, 55 mV or 100 mV range
is selected, the input signal (including the common
mode voltage and the amplifier offset voltage)
must not cause the 20X amplifier to saturate in ei-
ther its input stage or output stage. To prevent sat-
uration the absolute voltages on AIN+ and AIN-
must stay within the limits specified (refer to the
‘Analog Input’ table on page 3). Additionally, the
NBV
X20
M
U
X
AIN2+
AIN2-
AIN1+
AIN1-
CS5521
IN+
IN-
AIN4+
AIN4-
*
*
*
AIN1+
AIN1-
CS5523
M
U
X
IN+
IN-
IN+
IN-
Programmable
Gain
Differential
4th-order
delta-sigma
modulator
Digial
Filter
VREF+ VREF-
Figure 7. Multiplexer Configuration
NBV also supplies the negative supply
voltage for the coarse/fine change buffers