Op Amps for Everyone - The Repeater Builder's Technical ...

External Vref Circuits for ADCs/DACs
20
50
0
0
–20
–50
V OUT – dB
–40
–60
–80
Amplitude
Phase
–100
–150
–200
– Phase – Degrees
–100
–250
–120
10
Hz
100
Hz
1
kHz
10
kHz
100
kHz
1
MHz
10
MHz
–300
100
MHz
f – Frequency
Figure 13–9. Voltage Follower Frequency Response Plot
Figure 13–10 shows an external reference circuit that provides a wide adjustment range
of the ADC full-scale range. Resistors R281 and R282 play two roles in this circuit:
Form part of the low-pass filter used to rolloff noise
Isolate the ADC’s reference input load capacitance from the buffer op amp output
Potentiometer R247 sets the external common-mode voltage (V ocm ) for the differential
amplifier in Figure 13–11.
TLE2227
10 V
V REF
R 247
R 2
2.21 k
R 100
2 k
R 246
2 k
POT
R 9
1.1 k
POT R 1
1.05 k
TLE2227
2 k
POT
R 282
10
R 281
10
C 196
R 296
10
C 197
4.7 F
4.7 F
REF –
V OCM
OUTPUT
REF +
Figure 13–10.
External Voltage Reference Circuit for ADC/DAC.
Wireless Communication: Signal Conditioning for IF Sampling
13-17