Cirrus Logic CS5534-AS - Manual (Page 13)

CS5531/32/33/34-AS
DS289F5 13
instrumentation amplifier is typically 1200 pA
over -40°C to +85°C (MCLK=4.9152 MHz).
The common-mode plus signal range of the instru-
mentation amplifier is (VA-) + 0.7 V to (VA+) -
1.7 V.
Figure 4 illustrates the input models for the ampli-
fiers. The dynamic input current for each of the
pins can be determined from the models shown.
Note: The C=3.9pF and C = 14pF capacitors are
for input current modeling only. For physical
input capacitance see ‘Input Capacitance’
specification under Analog Characteristics.
2.1.1. Analog Input Span
The full-scale input signal that the converter can
digitize is a function of the gain setting and the ref-
erence voltage connected between the VREF+ and
VREF- pins. The full-scale input span of the con-
verter is [(VREF+) - (VREF-)]/(GxA), where G is
the gain of the amplifier and A is 2 for VRS = 0, or
A is 1 for VRS = 1. VRS is the Voltage Reference
Select bit, and must be set according to the differen-
tial voltage applied to the VREF+ and VREF- pins
on the part. See section 2.3.5 for more details.
After reset, the unity gain buffer is engaged. With a
2.5V reference this would make the full-scale input
range default to 2.5 V. By activating the instrumen-
tation amplifier (i.e. a gain setting other than 1) and
using a gain setting of 32, the full-scale input range
can quickly be set to 2.5/32 or about 78 mV. Note
that these input ranges assume the calibration regis-
ters are set to their default values (i.e. Gain = 1.0 and
Offset = 0.0).
2.1.2. Multiplexed Settling Limitations
The settling performance of the CS5531/32/33/34
in multiplexed applications is affected by the sin-
gle-pole, low-pass filter which follows the instru-
mentation amplifier (see Figure 3). To achieve data
sheet settling and linearity specifications, it is rec-
ommended that a 22 nF C0G capacitor be used.
Capacitors as low as 10 nF or X7R type capacitors
can also be used with some minor increase in dis-
tortion for AC signals.
2.1.3. Voltage Noise Density Performance
Figure 5 illustrates the measured voltage noise
density versus frequency from 0.025 Hz to 10 Hz
of a CS5532-AS. The device was powered with
±2.5 V supplies, using 30 Sps OWR, the 64x gain
range, bipolar mode, and with the input short bit
enabled.
AIN
Gain = 2, 4, 8, 16, 32, 64
C = 3 .9 pF
f =
Gain = 1
AIN
C = 14 pF
φ Coarse
1
φ Fine
1
V
≤ 12 mV
i = fV C
os
os n
V
≤ 8 mV
i = fV C
os
os n
MCLK
128
f =
MCLK
16
Figure 4. Input models for AIN+ and AIN- pins
1
10
100
1000
0.025 0.10 1.00 10.00
Frequency (Hz)
Figure 5. Measured Voltage Noise Density, 64x