ad773akd Analog Devices, Inc., ad773akd Datasheet - Page 9

• Manufacturer Part Number: ad773akd
• Description: 10-bit, 20 Msps Monolithic A/d Converter
• Manufacturer: Analog Devices, Inc.

REV. 0
EQUIVALENT REFERENCE INPUT CIRCUIT
The AD773A is designed to have a reference to analog input
voltage ratio of 2.5:1. When the AD773A is configured for
single-ended operation a 2.5 volt reference input establishes a
full-scale analog input voltage of 1 V p-p ( 500 mV with respect
to V
V
input voltage and analog input voltages can be changed. To
optimize the AD773A’s performance the 2.5:1 ratio should be
maintained. The simplified model of the AD773A’s reference
input circuit is shown in Figure 16.
The 2.5 V external reference is applied across resistor R1
producing a current which in turn generates a voltage V
Multiple reference currents are generated from V
used throughout the converter. R3 is used to cancel errors
induced by the input bias current of the REFGND buffer.
Figure 17 shows the SNR performance as the reference voltage
is varied from its nominal value of 2.5 V. The input full-scale
voltage is defined by the following equation,
The power dissipation is modulated by variations in the
reference voltage. Figure 18 shows the variation in power
dissipation versus reference voltage.
REF
INB
equal to 2.5 V and V
Figure 17. S/N+D vs. Reference Input Voltage,
f
CLK
). Although the AD773A is specified and tested with
Figure 16. Typical Reference Input Circuit
Input Full-Scale Voltage =
= 20 MSPS, f
60
55
50
45
40
35
30
1.0
REFGND
1.4
REFIN
REFERENCE INPUT VOLTAGE – V
AIN = –0.3dB
1.8
IN
IN
= 1 MHz
R3
equal to 500 mV the reference
2.2
AIN = –6dB
2.6
Reference Voltage
–5V
AD773A
3.0
A
2 .5
R1
R2
V
BIAS
3.4
BIAS
and are
3.8
BIAS
.
–9–
TRANSIENT RESPONSE
The fast settling input THA accurately converts full-scale input
voltage swings in under one clock cycle. The THA’s high
impedance, fast slewing performance is critical in multiplexed or
dc stepped (charge coupled devices, infrared detectors) systems.
Figure 19 show the AD773A’s settling performance with an
input signal stepped from –500 mV to 0 V. As can be seen, the
output code settles to its final value in under one clock cycle.
Figure 18. Power Dissipation vs. Reference Input
Voltage, f
1000
800
600
400
200
0
1.2
1.1
0.9
Figure 19. Typical AD773A Settling Time
0
1
2
CLOCK
2.1
2.2
= 20 MSPS
10
2.3
2.4
REFIN – V
TIME – ns
2.5
20
2.6
2.7
30
AD773A
2.8
2.9
3.0
40