AD9020

–8–

REV. C

Figure 5 shows a reference circuit that nulls out the offset errors

using two op amps, and provides appropriate voltage references

to the quarter-point taps. Feedback from the sense lines causes

the op amps to compensate for the offset errors. The two tran-

sistors limit the amount of current drawn directly from the op

amps; resistors at the base connections stabilize their operation.

The 10 k

Ω resistors (R1–R4) between the voltage sense lines

form an external resistor ladder; the quarter point voltages are

taken off this external ladder and buffered by an op amp. The

actual values of resistors R1–R4 are not critical, but they should

match well and be large enough (

≥10 kΩ) to limit the amount

of current drawn from the voltage sense lines.

a potentiometer) are chosen to adjust the quarter-point voltage

references, but are not necessary if R1–R4 match within 0.05%.

An alternative approach for defining the quarter-point references

of the resistor ladder is to evaluate the integral linearity error of

an individual device, and adjust the voltage at the quarter-points

to minimize this error. This may improve the low frequency ac

performance of the converter.

Performance of the AD9020 has been optimized with an analog

input voltage of

±1.75 V (as measured at ±V

input range is reduced below these values, relatively larger differ-

ential nonlinearity errors may result because of comparator

mismatches. As shown in Figure 4, performance of the converter

is a function of

0.4

32

38

44

50

56

62

0.6

0.8

1.0

1.2

1.4

1.8

2.0

1.6

5.0

6.0

7.0

8.0

9.0

10.0

Figure 4. SNR and ENOB vs. Reference Voltage

Applying a voltage greater than 4 V across the internal resistor

ladder will cause current densities to exceed rated values, and

may cause permanent damage to the AD9020. The design

of the reference circuit should limit the voltage available to

the references.

Analog Input Signal

The signal applied to ANALOG IN drives the inputs of 512

parallel comparator cells (see Figure 6). This connection typi-

cally has an input resistance of 7 k

Ω, and input capacitance of

45 pF. The input capacitance is nearly constant over the ana-

log input voltage range, as shown in the graph that illustrates

that characteristic.

The analog input signal should be driven from a low-distortion,

low-noise amplifier. A good choice is the AD9617, a wide

bandwidth, monolithic operational amplifier with excellent ac

and dc performance. The input capacitance should be isolated

by a small series resistor (24

Ω for the AD9617) to improve the

ac performance of the amplifier (see Figure 14).