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

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 dependance.

The above is indended 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 count on 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

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).

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