Rainbow Electronics ADC08060 - Manual (Page 16)

Applications Information
(Continued)
The RC at the amplifier output filters the clock rate energy
that comes out of the analog input due to the input sampling
circuit. The optimum time constant for this circuit depends
not only upon the amplifier and ADC, but also on the circuit
layout and board material. A resistor value should be chosen
between 18
Ω and 47Ω and the capacitor value chose ac-
cording to the formula
This will provide optimum SNR performance. Best THD per-
formance is realized when the capacitor and resistor values
are both zero. To optimize SINAD, reduce the capacitor or
resistor value until SINAD performance is optimized. That is,
until SNR = −THD. This value will usually be in the range of
40% to 65% of the value calculated with the above formula.
An accurate calculation is not possible because of the board
material and layout dependence.
The above is intended for oversampling or Nyquist applica-
tions. There should be no resistor or capacitor between the
ADC input and any amplifier for undersampling applications.
The circuit of Figure 4 has both gain and offset adjustments.
If you eliminate these adjustments normal circuit tolerances
may cause signal clipping unless care is exercised in the
worst case analysis of component tolerances and the input
signal excursion is appropriately limited to account for the
worst case conditions. Of course, this means that the de-
signer will not be able to depend upon getting a full scale
output with maximum signal input.
3.0 POWER SUPPLY CONSIDERATIONS
A/D converters draw sufficient transient current to corrupt
their own power supplies if not adequately bypassed. A
10 µF tantalum or aluminum electrolytic capacitor should be
placed within an inch (2.5 cm) of the A/D power pins, with a
0.1 µF ceramic chip capacitor placed within one centimeter
of the converter’s power supply pins. Leadless chip capaci-
tors are preferred because they have low lead inductance.
While a single voltage source is recommended for the V
A
and DR V
D
supplies of the ADC08060, these supply pins
should be well isolated from each other to prevent any digital
noise from being coupled into the analog portions of the
ADC. A choke or 27
Ω resistor is recommended between
these supply lines with adequate bypass capacitors close to
the supply pins.
As is the case with all high speed converters, the ADC08060
should be assumed to have little power supply rejection.
None of the supplies for the converter should be the supply
that is used for other digital circuitry in any system with a lot
of digital power being consumed. The ADC supplies should
be the same supply used for other analog circuitry.
No pin should ever have a voltage on it that is in excess of
the supply voltage or below ground by more than 300 mV,
not even on a transient basis. This can be a problem upon
application of power and power shut-down. Be sure that the
supplies to circuits driving any of the input pins, analog or
digital, do not come up any faster than does the voltage at
the ADC08060 power pins.
20006234
FIGURE 4. The input amplifier should incorporate some gain for best performance (see text).
ADC08060
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