adc12qs065civs National Semiconductor Corporation, adc12qs065civs Datasheet - Page 13

• Manufacturer Part Number: adc12qs065civs
• Description: Quad 12-bit 65 Msps A/d Converter With Lvds Serialized Outputs
• Manufacturer: National Semiconductor Corporation

Applications Information
Where dev is the angular difference in degrees between the
two signals having a 180˚ relative phase relationship to each
other (see Figure 3). Drive the analog inputs with a source
impedance less than 100Ω.
For differential operation, each analog input pin of the differ-
ential pair should have a peak-to-peak voltage just below the
reference voltage, V
each other and be centered around V
2.2.1 Single-Ended Operation
Performance with differential input signals is better than with
single-ended signals. For this reason, single-ended opera-
tion is not recommended. However, if single ended-operation
is required and the resulting performance degradation is
acceptable, one of the analog inputs should be connected to
the d.c. mid point voltage of the driven input. The peak-to-
peak differential input signal at the driven input pin should be
twice the reference voltage to maximize SNR and SINAD
performance (Figure 2b). For example, set V
bias V
to 1.5V.
Because very large input signal swings can degrade distor-
tion performance, better performance with a single-ended
input can be obtained by reducing the reference voltage
when maintaining a full-range output. Table 1 and Table 2
indicate the input to output relationship of the ADC12QS065.
TABLE 2. Input to Output Relationship – Single-Ended
TABLE 1. Input to Output Relationship – Differential
FIGURE 3. Angular Errors Between the Two Input
Signals Will Reduce the Output Level or Cause
IN
− to 1.0V and drive V
V
V
V
V
V
V
V
V
V
REF
REF
REF
V
V
V
V
CM
CM
REF / 4
CM
CM
CM
REF
CM
IN +
IN +
/ 2
/ 4
/ 2
−
−
+
+
−
REF
V
V
V
V
V
V
V
V
REF
REF
REF
V
V
V
V
REF / 2
CM
CM
CM
CM
, be 180 degrees out of phase with
Distortion
IN −
CM
IN −
CM
/ 4
/ 4
/ 2
Input
+
+
−
−
Input
IN
+ with a signal range of 0.5V
0000 0000 0000
0100 0000 0000
1000 0000 0000
0000 0000 0000
1100 0000 0000
Binary Output
Binary Output
1111 1111 1111
20106812
CM
.
(Continued)
REF
to 0.5V,
13
2.2.2 Driving the Analog Inputs
The V
an analog switch followed by a switched-capacitor amplifier.
The capacitance seen at the analog input pins changes with
the clock level, appearing as 8 pF when the clock is low, and
7 pF when the clock is high.
As the internal sampling switch opens and closes, current
pulses occur at the analog input pins, resulting in voltage
spikes at the signal input pins. As a driving amplifier attempts
to counteract these voltage spikes, a damped oscillation
may appear at the ADC analog input. Do not attempt to filter
out these pulses. Rather, use amplifiers to drive the
ADC12QS065 input pins that are able to react to these
puses and settle before the switch opens and another
sample is taken. The LMH6702 LMH6628, LMH6622 and the
LMH6655 are good amplifiers for driving the ADC12QS065.
To help isolate the pulses at the ADC input from the amplifier
output, use RCs at the inputs, as can be seen in Figure 4 .
These components should be placed close to the ADC in-
puts because the input pins of the ADC is the most sensitive
part of the system and this is the last opportunity to filter that
input.
For Nyquist applications the RC pole should be at the ADC
sample rate. The ADC input capacitance in the sample mode
should be considered when setting the RC pole. For wide-
band undersampling applications, the RC pole should be set
at about 1.5 to 2 times the maximum input frequency to
maintain a linear delay response.
A single-ended to differential conversion circuit is shown in
Figure 5. Table 3 gives resistor values for that circuit to
provide input signals in a range of 1.0V
differential input pins of the ADC12QS065.
2.2.3 Input Common Mode Voltage
The input common mode voltage, V
range of 0.5V to 2.0V and be a value such that the peak
excursions of the analog signal does not go more negative
than ground or more positive than 2.6V. The nominal V
should generally be about 1.5V, but VCOM12 or VCOM34
can be used as a V
0 - 0.25V
SIGNAL
RANGE
0 - 0.5V
±
TABLE 3. Resistor Values for Circuit of Figure 5
0.25V
IN
+ and the V
V
V
V
V
V
REF
REF
V
V
V
CM
CM
CM
REF
IN +
CM
100Ω
open
/ 2
/ 2
−
+
+
0Ω
R1
IN
CM
− inputs of the ADC12QS065 consist of
V
V
V
V
V
source.
CM
CM
CM
CM
openΩ
IN −
698Ω
0Ω
R2
0100 0000 0000
1000 0000 0000
1100 0000 0000
Binary Output
1111 1111 1111
124Ω
499Ω
100Ω
R3
CM
±
, should be in the
0.5V at each of the
1500Ω 1000Ω
1500Ω
698Ω
R4
www.national.com
R5, R6
499Ω
499Ω
CM