Wednesday, January 29, 2014

A Board IF Amplifier Gains

The Beagle Bone Black A-board 1MSPS ADC gain stages were characterized.  Measurements were taken using a 3mVpp input sine wave. The IF gain can be configured by the OpAmp used and feedback resistors employed.  The following table provides two measured configurations.  The gain of a stage (S0/S1) is controlled by the GPIO (G0/G1) selecting the mux setting controlling the feedback resistor.  The first stage input impedance is chosen to match the filter termination required (300 ohms) while the second stage is small (47 ohms) to allow a smaller feedback resistor (and noise).  In the table, the first stage feedback values of 3k or 4.7k are used in one configuration and 300 or 3k in the other.  Second stage values of 470 or 1k are used in both configurations.  For lower first stage gain configurations a unity gain stable OpAmp is used.  The table also shows the deviation from the simple gain calculation incorporating filter attenuation.


Unit
G0
G1
Calculated S0 gain (dB)
Calculated S1 gain (dB)
Calculated Module gain (db)
Measured gain (dB)
Measured-Calculated (dB)
1
0
0
20
20
37
37
-0.1
1
0
1
20
26
43
44
0.9
1
1
0
23
20
40
41
1.0
1
1
1
23
26
46
48
2.2
2
0
0
0
20
17
18
0.8
2
0
1
0
26
23
24
0.7
2
1
0
20
20
37
29
-8.4
2
1
1
20
26
43
35
-7.5
Calculated module gain is S0+S1-3dB for filter
Unit 1 uses AD4895 for both S0 and S1
Unit 1 settings are: S0=(3k || 4.7k)/300, S1=(470 || 1k)/47
Unit 2 uses AD4897 for S0 and ADA4895 for S1
Unit 2 settings are: S0=(300 || 3k)/300, S1=(470 || 1k)/47
Measurements using oscilloscope with 3mVpp input at 10.7MHz
 
The values match expectations (particularly given the error margin involved using a scope with small signals). The compensated OpAmp deviating more from the simple gain calculation is anticipated.

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