ADS-238Q
4
®®
Figure 6-2. Bias2 Voltage vrs Bias2 Current
Figure 4. Suggested Bias Current vrs Sample Rate
SAMPLE RATE
BIAS1
BIAS2
(MHz)
(µA)
(µA)
120
3.5
550
6.5
10
80
8.0
20
90
9.5
Table 1. Sample Rate Bias Current Settings
Common Mode Voltage Reference
The ADS-238Q has an internal common mode reference
voltage, VCM (pin 18). This reference voltage is typically one
half of the supply voltage and is capable of driving loads up
to 20µA. This reference is used to drive the center tap in
the transformer used in fully differential applications, or
provide level shifting in single-ended to differential
applications.
Bias Current Inputs
The bias currents shown in Table I, Sample Rate Bias
Current Settings, and Figure 4, Suggested Bias Currents
vrs Sample Rate, are designed to optimize performance of
the ADS-238Q for a given sample rate.
Figure 5, Bias Current Determination and Adjustment,
details the suggested circuits for measuring bias voltage
drops to calculate the bias currents. Adjustments to the bias
currents are made via the trim pots. The Bias1 voltage drop
is measured across TP1A and TP1B. The Bias2 voltage
drop is measured across TP2A and TP2B. Figures 6-1 and
6-2 show the relationship between the bias currents and
voltages when measured in the indicated circuit. The Bias1
and Bias2 pins should be by-passed with 0.01µF
capacitors.
Figure 5. Bias Current Determination and Adjustment
Figure 6-1. Bias1 Voltage vrs Bias1 Current
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
Bias2 (µA)
Sample Rate (MHz)
Bias1 (µA)
TP2A
TP1B
TP2B
100k
100k
100k
4.75k
500k
500k
TP1A
0.01µF
0.01µF
+
100k
100k
0.01µF
4.7µF
4.7V
Bias1
Bias2
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
0
30
60
90
120
150
180
Bias1
30
60
90
120
150
VBias1
2.19
2.53
2.79
3
3.22
IBias1 (µA)
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0
3
6
9
12
15
18
IBias2
3
6
9
12
15
VBias2
0.6975
0.7535
0.796
0.8295
0.8595
IBias2 (µA)
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