ACCURATELY EVALUATING THE ADC

If a signal that is spectrally impure is presented to the ADC,

the output from the ADC cannot be pure. Nearly all signal

generators in use today produce signals that are not spec-

trally pure enough to adequately evaluate present-day

ADCs. This is especially true at higher frequencies and at

high resolutions.

To ensure that the signal you are presenting to the ADC be-

ing evaluated is spectrally pure, use a bandpass filter be-

tween the signal generator and the ADC input. One such

possible filter is the elliptic filter shown in Figure 8.

In addition to being used to eliminate undesired frequencies

from a desired signal, this filter can be used to filter a square

wave, reducing 3rd and higher harmonics to negligible lev-

els.

When evaluating dynamic performance of an ADC, repeat-

ability of measurements could be a problem unless coherent

sampling is used.

and ADC08351 evaluation system is available that can sim-

plify evaluation of thsi product.

7.0 COMMON APPLICATION PITFALLS

Driving the inputs (analog or digital) beyond the power

supply rails. For proper operation, all inputs should not go

more than 300 mV beyond the supply rails. That is, more

than 300 mV below the ground pins or 300 mV above the

supply pins. Exceeding these limits on even a transient basis

may cause faulty or erratic operation. It is not uncommon for

high speed digital circuits (e.g., 74F and 74AC devices) to

exhibit undershoot that goes more than a volt below ground

or above the power supply. Since these conditions are of

very short duration with very fast rise and fall times, they can

DS100895-29

FIGURE 7. Inverting Circuit with Bias Adjust

DS100895-31

FIGURE 8. This elliptic filter has a cutoff frequency of about 11MHz and is suitable for input frequencies of 5MHz to

10MHz. It should be driven by a generator of 75

Ω source impedance and teminated with 75Ω. This termination may

be provided by the ADC evaluation circuit.

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