AD7575

–8–

REV. B

INTERNAL/EXTERNAL CLOCK

The AD7575 can be used with its own internal clock or with an

externally applied clock. In either case, the clock signal appear-

ing at the CLK pin is divided internally by two to provide an

internal clock signal for the AD7575. A single conversion lasts

for 20 input clock cycles (10 internal clock cycles).

INTERNAL CLOCK

Clock pulses are generated by the action of the external capaci-

discharging through an internal switch. When a conversion is

complete, the internal clock stops operating. In addition to

conversion, the internal clock also controls the automatic inter-

nal reset of the SAR. This reset occurs at the start of each con-

version cycle during the first internal clock pulse.

Nominal conversion times versus temperature for the recom-

AMBIENT TEMPERATURE – C

14

7

–55

+125

–25

0

+25

+50

+75

+100

13

11

10

9

8

12

Figure 13. Typical Conversion Times vs. Temperature

Using Internal Clock

The internal clock is useful because it provides a convenient

clock source for the AD7575. Due to process variations, the

vary from device to device by up to

±50%. For this reason it is

recommended that an external clock be used in the following

situations:

1. Applications requiring a conversion time that is within 50% of

5

µs, the minimum conversion time for specified accuracy. A

clock frequency of 4 MHz at the CLK pin gives a conversion

time of 5

µs.

2. Applications where time related software constraints cannot

accommodate time differences that may occur due to unit to

unit clock frequency variations or temperature.

EXTERNAL CLOCK

The CLK input of the AD7575 may be driven directly from

74 HC, 4000B series buffers (such as 4049) or from LS TTL

with a 5.6 k

Ω pull-up resistor. When conversion is complete, the

internal clock is disabled even if the external clock is still ap-

plied. This means that the external clock can continue to run

between conversions without being disabled. The mark/space

ratio of the external clock can vary from 70/30 to 30/70.

The AD7575 is specified for operation at a 5

µs conversion rate;

with a 4 MHz input clock frequency. If the part is operated at

slower clock frequencies, it may result in slightly degraded accu-

racy performance from the part. This is a result of leakage ef-

fects on the hold capacitor. Figure 14 shows a typical plot of

accuracy versus conversion time for the AD7575.

CONVERSION TIME –

s

2.5

5

10000

50

500

2.0

1.5

1.0

0.5

0

100

1000

5000

10

AD7575KN

Figure 14. Accuracy vs. Conversion Time