AD9501

REV. A

–9–

This delay matching is often difficult when using high speed,

high-pin-count testers because lead length and circuit

impedance can change when the tester setup is changed for

different types of devices. The skew which might result from

these changes can be compensated by using AD9501 units as

shown in Figure 5.

When deskewing multiple signal paths, a single stimulus pulse is

applied to all inputs of the AD9501s which are used. The delay

for each signal path is then measured by the tester’s delay

measurement circuit. Using a closed loop technique, all delays

are equalized by changing the digital value held in the register of

each AD9501. Once all delays have been matched to the desired

tolerance, the calibration loop is opened; and the tester is ready

to test the new type of device.

Digitally Programmable Oscillator

Two AD9501s can be configured as an stable oscillator, as

shown in Figure 6.

triggered from a common clock signal. Their outputs go to the

inputs of an RS flip-flop. A digital delay value is applied as an

input to each with AD9501 #2 typically having a larger value

than AD9501 #1.

As shown by the timing portion of the diagram, changing the

delay value from one clock cycle to the next generates a pseudo-

random pulse whose leading and trailing edge delays are con-

trolled relative to Clock In. The dashed lines illustrate how the

programmed delays of the AD9501 components control both

the timing and width of the generator output.

can be determined as follows:

f

= f

CLOCK IN

and:

Figure 6. Digitally Programmable Oscillator

Delay through each side of the oscillator is determined by the

applied to either AD9501 decreases frequency, just as

increasing RC decreases frequency in an analog ring oscillator.

Using a pair of AD9501 Delay Generators as shown allows the

user great flexibility because both the frequency and the duty

cycle of the oscillator are easily controlled.

Frequency of the oscillator output can be established with the

equation:

f

=1/(2t

AD9502 #2, respectively.

Programmable Pulse Generator

In this application, shown in Figure 7, two AD9501 units are

set for the same full-scale delay range, their minimum

propagation delays will be approximately the same, and the

pulse width will be approximately equal to the difference in

programmed delays.

Digital Delay Detector

An unknown digital delay can be measured by applying a

repetitive clock to the circuit shown in Figure 8.

The pictured delay detector works in a manner similar to a

successive approximation ADC; in this circuit, however, a

D-type flip-flop replaces the ADC’s voltage comparator.

To calibrate the circuit, short out the unknown delay and apply

the clock input to both AD9501 units.