Analog Devices AD8209A-EVALZ, User Manual

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AD8209A-EVALZ User Guide UG-1024
Rev. 0 | Page 5 of 8
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:
Gain
=
Ω +
×
k 100
14
R6
R6
R6 = 100 kΩ ×
Gain
Gain
−
14
GND
DNC
–IN
+IN
A1
+V
S
A2
OUT
AD8209A
5V
V
DIFF
V
CM
NOTES
1. DNC = DO NOT CONNECT.
R6
OUTPUT
+
–
+
–
14790-
007
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):
Gain =
Ω −
×
k 100 7
7 14
R
R
R7 = 100 kΩ ×
14
−
Gain
Gain
GND
DNC
–IN
+IN
A1 A2
OUT
AD8209A
5V
V
DIFF
V
CM
NOTES
1. DNC = DO NOT CONNECT.
R7
POINT X
(SEE TEXT)
OUTPUT
+
–
+
–
+V
S
14790-
008
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
C
) of the
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.
GND
DNC
–IN
+IN
A1
+V
S
A2
OUT
AD8209A
5V
V
DIFF
V
CM
C4
NOTES
1. DNC = DO NOT CONNECT.
OUTPUT
f
C
=
1
2πC10
5
C IN FARADS
+
–
+
–
14790-
009
Figure 9. Single-Pole, Low-Pass Filter Using the Internal 100 kΩ Resistor
PROVISIONS FOR EXTERNAL COMPONENTS
There are provisions available on the AD8209A evaluation
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)