ad8200r Analog Devices, Inc., ad8200r Datasheet - Page 7

• Manufacturer Part Number: ad8200r
• Description: High Common-mode Voltage, Single Supply Difference Amplifier
• Manufacturer: Analog Devices, Inc.

Internal Signal Overload Considerations
When configuring gain for values other than 20, the maximum
input voltage with respect to the supply voltage and ground
must be considered, since either the preamplifier or the output
buffer will reach its full-scale output (approximately V
with large differential input voltages. The input of the AD8200
is limited to (V
preamplifier, with its fixed gain of ×10, reaches its full-scale
output before the output buffer. For gains greater than 10, the
swing at the buffer output reaches its full-scale first and limits
the AD8200 input to (V
LOW-PASS FILTERING
In many transducer applications it is necessary to filter the sig-
nal to remove spurious high-frequency components, including
noise, or to extract the mean value of a fluctuating signal with a
peak-to-average ratio (PAR) greater than unity. For example, a
full-wave rectified sinusoid has a PAR of 1.57, a raised cosine
has a PAR of 2, and a half-wave sinusoid has a PAR of 3.14.
Signals having large spikes may have PARs of 10 or more.
When implementing a filter, the PAR should be considered so
the output of the AD8200 preamplifier (A1) does not clip before
A2, since this nonlinearity would be averaged and appear as an
error at the output. To avoid this error, both amplifiers should
be made to clip at the same time. This condition is achieved
when the PAR, is no greater than the gain of the second ampli-
fier (2 for the default configuration). For example, if a PAR of 5
is expected, the gain of A2 should be increased to 5.
Low-pass filters can be implemented in several ways using the
features provided by the AD8200. In the simplest case, a single-
pole filter (20 dB/decade) is formed when the output of A1 is
connected to the input of A2 via the internal 100 kΩ resistor by
strapping Pins 3 and 4, and a capacitor added from this node to
ground, as shown in Figure 8. If a resistor is added across the
capacitor to lower the gain, the corner frequency will increase; it
should be calculated using the parallel sum of the resistor and
100 kΩ.
If the gain is raised using a resistor, as shown in Figure 8, the
corner frequency is lowered by the same factor as the gain is
raised. Thus, using a resistor of 200 kΩ (for which the gain
would be doubled) the corner frequency is now 0.796 Hz-µF,
(0.039 µF for a 20 Hz corner frequency.)
Ω
V
CM
S
V
V
DIFF
DIFF
– 0.2) ÷ 10, for overall gains ≤10, since the
2
2
NC = NO CONNECT
S
– 0.2) ÷ G, where G is the overall gain.
+IN NC +V
–IN GND A1
AD8200
5V
S
OUT
A2
C
F
C IN FARADS
C
OUT
=
2 C10
1
S
5
– 0.2 V)
A 2-pole filter (with a roll-off of 40 dB/decade) can be imple-
mented using the connections shown in Figure 9. This is a
Sallen-Key form based on a ×2 amplifier. It is useful to remem-
ber that a 2-pole filter with a corner frequency f
filter with a corner at f
frequency (f
(f
tor value shown, and equal capacitors (Figure 9), the corner
frequency is conveniently scaled at 1 Hz-µF (0.05 µF for a 20 Hz
corner). A maximally flat response occurs when the resistor is
lowered to 196 kΩ and the scaling is then 1.145 Hz-µF. The
output offset is raised by about 5 mV (equivalent to 250 V at
the input pins).
2
/f
1
). This is illustrated in Figure 10. Using the standard resis-
40LOG (f
V
CM
2
/f
2
1
2
)
/f
1
V
V
). The attenuation at that frequency is 40 Log
DIFF
DIFF
2
2
NC = NO CONNECT
A 1-POLE FILTER, CORNER f
A 2-POLE FILTER, CORNER f
THE SAME ATTENUATION –40LOG (f
AT FREQUENCY f
FREQUENCY
20dB/DECADE
1
have the same attenuation at the
f
1
+IN NC +V
–IN GND A1
2
2
AD8200
/f
1
5V
f
S
2
C
1
2
OUT
A2
, AND
, HAVE
255k
40dB/DECADE
2
F
/f
C
1
)
= 1Hz – F
2
f
AD8200
2
2
and a 1-pole
/f
C
1
OUT