AD9020

–8–

REV. A

USING THE AD9020

Voltage References

The AD9020 requires that the user provide two voltage refer-

an internal resistor ladder (nominally 37

Ω) and set the analog

input voltage range of the converter. The voltage references

should be driven from a stable, low impedance source. In addi-

tion to these two references, three evenly spaced taps on the re-

reference to these quarter points on the resistor ladder will im-

prove the integral linearity of the converter and improve ac per-

formance. (AC and dc specifications are tested while driving the

quarter points at the indicated levels.) The figure below is not

intended to show the transfer function of the ADC, but illus-

trates how the linearity of the device is affected by reference

voltages applied to the ladder.

Effect of Reference Taps on Linearity

Resistance between the reference connections and the taps of

the first and last comparators causes offset errors. These errors,

called “top and bottom of the ladder offsets,” can be nulled by

reference voltages. Current through the sense lines should be

limited to less than 100

µA. Excessive current drawn through

the voltage sense lines will affect the accuracy of the sense line

voltage.

The next page shows a reference circuit which nulls out the off-

set 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 transistors 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 volt-

age 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.

be a potentiometer) are chosen to adjust the quarter-point volt-

age references, but are not necessary if R1–R4 match within

0.05%.

An alternative approach for defining the quarter-point refer-

ences 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

log input range is reduced below these values, relatively larger

differential nonlinearity errors may result because of comparator

mismatches. As shown in the figure below, performance of the

converter is a function of

±V

32

38

44

50

56

62

10.0

9.0

8.0

7.0

6.0

5.0

0.4

0.8

1.0

1.4

1.8

2.0

0.6

1.2

1.6

AD9020 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 Equivalent Analog Input figure).

This connection typically has an input resistance of 7 k

Ω, and

input capacitance of 45 pF. The input capacitance is nearly

constant over the analog input voltage range, as shown in the

graph which 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 band-

width, 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 AD9020/PCB Evaluation

Board Block Diagram).