AD650 datasheet(10/21 Pages) AD

10 / 21 page

Data Sheet

AD650

Rev. E | Page 9 of 20

If the approximate amount of noise that appears on CINT is known

(VNOISE), then the value of CINT can be checked using the following

inequality:

NOISE

INT











(8)

For example, consider an application calling for a maximum

frequency of 75 kHz, a 0 V to 1 V signal range, and supply

voltages of only ±9 V. The component selection guide of Figure 9

is used to select 2.0 kΩ for RIN and 1000 pF for COS. This results

in a one-shot time period of approximately 7 μs. Substituting

75 kHz into Equation 7 yields a value of 1300 pF for CINT. When

the input signal is near zero, 1 mA flows through the integration

capacitor to the switched current sink during the reset phase,

causing the voltage across CINT to increase by approximately 5.5 V.

Because the integrator output stage requires approximately 3 V

headroom for proper operation, only 0.5 V margin remains for

integrating extraneous noise on the signal line. A negative noise

pulse at this time could saturate the integrator, causing an error

in signal integration. Increasing CINT to 1500 pF or 2000 pF

provides much more noise margin, thereby eliminating this

potential trouble spot.

1MHz

100kHz

10kHz

100

1000

COS (pF)

INPUT

RESISTOR

16.9k

20k

40.2k

100k

Figure 9. Full-Scale Frequency vs. COS

100

1000

ONE SHOT CAPACITOR

COS (pF)

1000

INPUT

RESISTOR

16.9k

40.2k

100k

20k

Figure 10. Typical Nonlinearity vs. COS

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