AD871

REV. A

–11–

The wide input bandwidth and superior dynamic performance of

the input THA make the AD871 suitable for sampling inputs at

frequencies up to the Nyquist Rate. The input THA is designed

to recover rapidly from input overdrive conditions, returning

from a 50% overdrive in less than 100 ns.

Because of the THA’s exceptionally wide input bandwidth, some

users may find the AD871 is sensitive to noise at frequencies

from 10 MHz to 50 MHz that other converters are incapable of

responding to. This sensitivity can be mitigated by careful use of

the differential inputs (see previous paragraphs). Additionally,

Figure 18 shows how a small capacitor (10 pF – 20 pF for 50

Ω

reduce high frequency noise in applications where limiting the

input bandwidth is acceptable.

1

2

AD871

1V

10 OR 20pF

Figure 18. Optional High Frequency Noise Reduction

The AD871 will contribute its own wideband thermal noise. As

a result of the integrated wideband noise (0.17 LSB rms,

referred-to-input), applying a dc analog input may produce more

than one code at the output. A histogram of the ADC output

codes, for a dc input voltage, will be between 1 and 3 codes

wide, depending on how well the input is centered on a given

code and how many samples are taken. Figure 8 shows a typical

AD871 code histogram, and Figure 9 illustrates the AD871’s

transition noise.

REFERENCE INPUT

The nominal reference input should be 2.5 V, taken with respect

to REFERENCE GROUND (REF GND). Figure 19 illustrates

the equivalent model for the reference input: there is no clock or

signal-dependent activity associated with the reference input cir-

cuitry, therefore no “kickback” into the reference.

1

2

REF IN

REF GND

AD871

5k

( 20%)

Figure 19. Equivalent Reference Input Circuit

However, in order to realize the lowest noise performance of the

AD871, care should be taken to minimize noise at the reference

input.

The AD871’s reference input impedance is equal to 5 k

Ω (±20%),

and its effective noise bandwidth is 10 MHz, with a referred-

to-input noise gain of 0.8. For example, the internal reference,

with an rms noise of 28

µV (using an external 1 µF capacitor),

contributes 24

µV (0.05 LSB) of noise to the transfer function

of the AD871.

The full-scale peak-to-peak input voltage is a function of the ref-

erence voltage, according to the equation:

× (V

Note that the AD871’s performance was optimized for a 2.5 V

reference input: performance may degrade somewhat for other

reference voltages. Figure 20 illustrates the S/(N+D) perfor-

mance vs. reference voltage for a 1 MHz, –0.5 dB input signal.

Note also that if the reference is changed during a conversion,

all three conversions in the pipeline will be invalidated.

1.5

2

3.5

3

2.5

70

60

50

REFERENCE INPUT VOLTAGE – Volts

Figure 20. S/(N+D) vs. Reference Input Voltage,

Table II summarizes various 2.5 V references suitable for use

with the AD871, including the onboard bandgap reference (see

REFERENCE OUTPUT section).

Table II. Suitable 2.5 V References

Drift (PPM/ C)

Initial Accuracy %

REF-43B

6 (max)

0.2

AD680JN

10 (max)

0.4

Internal

30 (typ)

0.4

If an external reference is connected to REF IN, REF OUT

must be connected to +5 V. This should lower the current in

REF GND to less than 350

µA and eliminate the need for a

1

µF capacitor, although decoupling the reference for noise

reduction purposes is recommended.

Alternatively, Figure 21 shows how the AD871 may be driven

from other references by use of an external resistor. The exter-

nal resistor forms a resistor divider with the on-chip 5 k

Ω resis-

tor to realize 2.5 V at the reference input pin (REF IN). A trim

potentiometer is needed to accommodate the tolerance of the

AD871’s 5 k

Ω resistor.