ADS-238Q Datel, Inc., ADS-238Q Datasheet - Page 4

• Manufacturer Part Number: ADS-238Q
• Description: 12-bit, 20mhz, Low Power Sampling A/D Converters
• Manufacturer: Datel, Inc.

ADS-238Q
Common Mode Voltage Reference
4.7V
SAMPLE RATE
The ADS-238Q has an internal common mode reference
voltage, V
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.
4.7µF
90
80
70
60
50
40
30
20
10
Figure 5. Bias Current Determination and Adjustment
0
(MHz)
Figure 4. Suggested Bias Current vrs Sample Rate
10
20
0
+
1
5
Table 1. Sample Rate Bias Current Settings
CM
0.01µF
(pin 18). This reference voltage is typically one
5
TP1A
TP1B
500k
Sample Rate (MHz)
500k
100k
100k
BIAS1
(µA)
20
50
80
90
10
4.75k
100k
100k
TP2A
TP2B
100k
15
BIAS2
Bias1 (µA)
Bias2 (µA)
0.01µF
0.01µF
(µA)
3.5
6.5
8.0
9.5
Bias2
Bias1
20
4
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.
0.90
0.85
0.80
0.75
0.70
0.65
0.60
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
0
0
Figure 6-1. Bias1 Voltage vrs Bias1 Current
Figure 6-2. Bias2 Voltage vrs Bias2 Current
30
3
60
6
IBias1 (µA)
IBias2 (µA)
90
9
®
120
12
Bias1
IBias2
120
150
30
60
90
12
15
3
6
9
150
15
VBias1
VBias2
0.6975
0.7535
0.8295
0.8595
0.796
2.19
2.53
2.79
3.22
3
180
18
®