Evaluation Board Configuration; Gain Adjustment Options; Gain Values Less Than 14; Gains Values Greater Than 14 - Analog Devices AD8209A-EVALZ User Manual

AD8209A-EVALZ User Guide

EVALUATION BOARD CONFIGURATION

GAIN ADJUSTMENT OPTIONS

By default, the gain of the preamplifier and the buffer are 7 V/V
and 2 V/V, respectively, resulting in a composite gain of 14 V/V.
The gain can be lowered or raised with the addition of external
resistors.

Gain Values Less Than 14

The preamplifier has an output resistance of 100 kΩ. Populating
an external resistor, R6, on the evaluation board decreases the
gain of the AD8209A. To calculate the effective gain or the
value of the required external resistor (R6), refer to the
following equations:
×
14 R6
Gain =
+ Ω
R6 100 k
Gain
R6 = 100 kΩ ×
−
14 Gain
+IN
+
AD8209A
V
DIFF
–
–IN GND
+
V
CM
–
NOTES
1. DNC = DO NOT CONNECT.
Figure 7. Adjusting for Gains Less Than 14

Gains Values Greater Than 14

Populating R7 on the board increases the gain of the AD8209A.
An overall gain as high as 50 is achievable, but note that the
accuracy of the gain becomes critically dependent on the
resistor value at high gains. See the following equations to
calculate the effective gain or to calculate the value of the
external resistor (R7):
×
14 R 7
Gain =
− Ω
R 7 100 k
Gain
R7 = 100 kΩ ×
−
Gain 14
5V
OUTPUT
+V DNC OUT
S
A1 A2
R6
V
DIFF
V
CM
NOTES
1. DNC = DO NOT CONNECT.
Figure 8. Adjusting for Gains Greater Than 14

LOW-PASS FILTERING

Implement the simplest low-pass filter by adding a capacitor to
C4. This forms a single-pole filter (20 dB/decade) together with
the internal 100 kΩ resistor. Note that the corner frequency
changes after R4, R6, or R7 are populated. Adding a resistor to
R6 to lower the gain increases the corner frequency (f
filter. Consequently, populating R7 to increase the gain lowers
the corner frequency of the filter by the same factor by which
the gain is raised.
V
DIFF
V
CM
NOTES
1. DNC = DO NOT CONNECT.
Figure 9. Single-Pole, Low-Pass Filter Using the Internal 100 kΩ Resistor

PROVISIONS FOR EXTERNAL COMPONENTS

There are provisions available on the
board for the following components:
•
Sense resistor (R1)
•
Input common-mode filters (C1 and C3)
•
Input differential filter (C2)
•
Low-pass, single-pole filter option (C4)
•
Load capacitor (C7)
•
Option for lowering the gain (R6)
•
Option for increasing the gain (R7)
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5V
OUTPUT
+IN +V DNC OUT
S
+
AD8209A
R7
–
–IN GND A1 A2
+
POINT X
(SEE TEXT)
–
5V
OUTPUT
+IN +V DNC OUT
S
+
f
=
AD8209A 2πC10
C
–
C IN FARADS
–IN GND A1 A2
+
C4
–
AD8209A
UG-1024
) of the
C
1
5
evaluation