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ADC08D1500EVAL Datasheet

  • ADC08D1500EVAL

  • High Performance, Low Power, Dual 8-Bit, 1.5 GSPS A/D Conver...

  • 861.71KB

  • 33页

  • NSC

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ADC08D1500
2.0 Applications Information
(Continued)
Note that a precise d.c. common mode voltage must be
present at the ADC inputs. This common mode voltage,
V
CMO
, is provided on-chip when a.c. input coupling is used
and the input signal is a.c. coupled to the ADC.
When the inputs are a.c. coupled, the V
CMO
output
must
be
grounded, as shown in
Figure 11.
This causes the on-chip
V
CMO
voltage to be connected to the inputs through on-chip
50k-Ohm resistors.
IMPORTANT NOTE:
An Analog input channel that is not
used (e.g. in DES Mode) should be left floating when the
inputs are a.c. coupled. Do not connect an unused analog
input to ground.
20152155
FIGURE 12. Example of Servoing the Analog Input with
V
CMO
One such circuit should be used in front of the V
IN
+ input and
another in front of the V
IN
鈭?input. In that figure, R
D1
, R
D2
and
R
D3
are used to divide the V
CMO
potential so that, after being
gained up by the amplifier, the input common mode voltage
is equal to V
CMO
from the ADC. R
D1
and R
D2
are split to
allow the bypass capacitor to isolate the input signal from
V
CMO
. R
IN
, R
D2
and R
D3
will divide the input signal, if nec-
essary. Capacitor "C" in
Figure 12
should be chosen to keep
any component of the input signal from affecting V
CMO
.
Be sure that the current drawn from the V
CMO
output does
not exceed 100 碌A.
The Input impedance in the d.c. coupled mode (V
CMO
pin not
grounded) consists of a precision 100鈩?resistor between
V
IN
+ and V
IN
鈭?and a capacitance from each of these inputs
to ground. In the a.c. coupled mode the input appears the
same except there is also a resistor of 50K between each
analog input pin and the V
CMO
potential.
Driving the inputs beyond full scale will result in a saturation
or clipping of the reconstructed output.
2.2.1 Handling Single-Ended Input Signals
There is no provision for the ADC08D1500 to adequately
process single-ended input signals. The best way to handle
single-ended signals is to convert them to differential signals
before presenting them to the ADC. The easiest way to
accomplish single-ended to differential signal conversion is
with an appropriate balun-connected transformer, as shown
in
Figure 13.
20152144
FIGURE 11. Differential Input Drive
When the d.c. coupled mode is used, a common mode
voltage must be provided at the differential inputs. This
common mode voltage should track the V
CMO
output pin.
Note that the V
CMO
output potential will change with tem-
perature. The common mode output of the driving device
should track this change.
IMPORTANT NOTE:
An analog input channel that is not
used (e.g. in DES Mode) should be tied to the V
CMO
voltage
when the inputs are d.c coupled. Do not connect unused
analog inputs to ground.
Full-scale distortion performance falls off rapidly as the
input common mode voltage deviates from V
CMO
. This is
a direct result of using a very low supply voltage to
minimize power. Keep the input common voltage within
50 mV of V
CMO
.
Performance is as good in the d.c. coupled mode as it is
in the a.c. coupled mode, provided the input common
mode voltage at both analog inputs remain within 50 mV
of V
CMO
.
If d.c. coupling is used, it is best to servo the input common
mode voltage, using the V
CMO
pin, to maintain optimum
performance. An example of this type of circuit is shown in
Figure 12.
20152143
FIGURE 13. Single-Ended to Differential signal
conversion with a balun-connected transformer
The 100 Ohm external resistor placed accross the output
terminals of the balun in parallel with the ADC08D1500鈥檚
on-chip 100 Ohm resistor makes a 50 Ohms differential
impedance at the balun output. Or, 25 Ohms to virtual
ground at each of the balun output terminals.
Looking into the balun, the source sees the impedance of the
first coil in series with the impedance at the output of that
coil. Since the transformer has a 1:1 turns ratio, the imped-
ance across the first coil is exactly the same as that at the
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