AD8555 Analog Devices, AD8555 Datasheet - Page 25

• Manufacturer Part Number: AD8555
• Description: Zero-Drift, Single-Supply, Sensor Signal Amplifier with Digitally Programmable Gain and Offset
• Manufacturer: Analog Devices

FILTERING FUNCTION
The AD8555’s FILT/DIGOUT pin can be used to create a simple
low-pass filter. The AD8555’s internal 18 kΩ resistor can be
used with an external capacitor for this purpose. Typical
responses of the AD8555, configured for a gain of 70 and gain
of 1280, are shown in Figure 54 and Figure 55, respectively. This
filtering feature can be used to pass the signals within the filter’s
pass band while limiting the out-of-band signals bandwidth
and, therefore, reducing the noise of the overall solution.
Figure 55. Typical Response of the AD8555 at FILT/DIGOUT Pin (Gain = 1280)
Figure 54. Typical Response of the AD8555 at FILT/DIGOUT Pin (Gain = 70)
40
20
60
40
20
0
0
10
10
V
C
OUT
C
FILTER
FILTER
Figure 53. AD8555 Configured to Filter Noise
VDD
V
= 0.100 µ F
IN
100
100
C
C
FILTER
FILTER
1
2
3
4
VDD
DIGIN
VNEG
FILT/DIGOUT
= 0.010 µ F
= 0.010 µ F
AD8555
1k
1k
VCLAMP
VOUT
VPOS
C
VSS
FILTER
C
FILTER
C
FILTER
8
7
6
5
= 0.001 µ F
10k
= 0.001 µ F
10k
= 0.100 µ F
VSS
VDD
100k
50k
Rev. 0 | Page 25 of 28
DRIVING CAPACITIVE LOADS
The AD8555 can drive large capacitive loads. This feature is
useful when the amplifier, placed close to the sensor, has to
drive long cables. Most instrumentation amplifiers have diffi-
culty driving capacitance due to the degradation of the phase
margin caused by the additional phase lag from the capacitive
load. Higher capacitance at the output can increase the amount
of overshoot and ringing in the amplifier’s step response and
could even affect the stability of the device. Additionally, the
value of the capacitive load that an amplifier can drive before
oscillation varies with gain, supply voltage, input signal, and
temperature. Figure 57 and Figure 58 show the overshoot
response of AD8555 versus the capacitive load with a different
value isolation resistor (R
ers, the AD8555 responds with overshoot when driving large C
but after a point (approximately 22 nF), the overshoot decreases.
This is because the pole created by C
ever, at some point, the pole is farther in than the pole setting of
the buffer amplifier and is ignored by AD8555.
60
50
40
30
20
10
C
0
0.1
FILTER
VDD
V
S
OUTPUT
BUFFER
Figure 56. Test Circuit for Driving Capacitive Loads
= ±2.5V
Figure 57. Positive Overshoot Graph vs. C
1
2
3
4
R
VDD
DIGIN
VNEG
FILT/DIGOUT
S
R
S
AD8555
= 100
LOAD CAPACITANCE (nF)
C
1
L
S
) in Figure 56. Similar to all amplifi-
= 1nF
VCLAMP
VOUT
VPOS
VSS
R
8
7
6
5
S
L
= 50
dominates at first; how-
VSS
10
VDD
R
R
S
S
= 20
R
L
C
R
S
L
S
= 10
= 0
V
AD8555
OUT
100
L
,