AD7607 datasheet(20/32 Pages) AD

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AD7608

Data Sheet

Rev. A | Page 20 of 32

Analog Input Antialiasing Filter

An analog antialiasing filter (a second-order Butterworth) is also

provided on the AD7608. Figure 34 and Figure 35 show the

frequency and phase response, respectively, of the analog

antialiasing filter. In the ±5 V range, the −3 dB frequency is

typically 15 kHz. In the ±10 V range, the −3 dB frequency is

typically 23 kHz.

–5

–10

–15

–20

–25

–30

–35

–40

100

10k

100k

INPUT FREQUENCY (Hz)

±10V RANGE

±5V RANGE

AVCC, VDRIVE = 5V

fSAMPLE = 200kSPS

TA = 25°C

±10V RANGE

0.1dB

3dB

–40

10,303Hz 24,365Hz

+25

9619Hz

23,389Hz

+85

9326Hz

22,607Hz

±5V RANGE

0.1dB

3dB

–40

5225Hz

16,162Hz

+25

5225Hz

15,478Hz

+85

4932Hz

14,990Hz

Figure 34. Analog Antialiasing Filter Frequency Response

–2

100

100k

10k

INPUT FREQUENCY (Hz)

AVCC, VDRIVE = 5V

fSAMPLE = 200kSPS

TA = 25°C

±5V RANGE

±10V RANGE

Figure 35. Analog Antialiasing Filter Phase Response

Track-and-Hold Amplifiers

The track-and-hold amplifiers on the AD7608 allow the ADC

to accurately acquire an input sine wave of full-scale amplitude

to 18-bit resolution. The track-and-hold amplifiers sample

their respective inputs simultaneously on the rising edge of

CONVST x. The aperture time for track-and-hold (that is, the

delay time between the external CONVST x signal and the

track-and-hold actually going into hold) is well matched, by design,

across all eight track-and-holds on one device and from device

to device. This matching allows more than one AD7608 device

to be sampled simultaneously in a system.

The end of the conversion process across all eight channels is

indicated by the falling edge of BUSY; and it is at this point that the

track-and-holds return to track mode, and the acquisition time

for the next set of conversions begins.

The conversion clock for the part is internally generated, and

the conversion time for all channels is 4 µs on the AD7608. The

BUSY signal returns low after all eight conversions to indicate the

end of the conversion process. On the falling edge of BUSY, the

track-and-hold amplifiers return to track mode. New data can

be read from the output register via the parallel, parallel byte, or

serial interface after BUSY goes low; or, alternatively, data from

the previous conversion can be read while BUSY is high. Reading

data from the AD7608 while a conversion is in progress has little

effect on performance and allows a faster throughput to be

achieved. In parallel mode at VDRIVE > 3.3 V, the SNR is reduced

by ~1.5 dB when reading during a conversion.

ADC TRANSFER FUNCTION

The output coding of the AD7608 is twos complement. The

designed code transitions occur midway between successive

integer LSB values, that is, 1/2 LSB, 3/2 LSB. The LSB size is

FSR/262,144 for the AD7608. The ideal transfer characteristic

for the AD7608 is shown in Figure 36.

011...111

011...110

000...001

000...000

111...111

100...010

100...001

100...000

–FS + 1/2LSB

0V – 1LSB

+FS – 3/2LSB

ANALOG INPUT

+FS

MIDSCALE

–FS

LSB

±10V RANGE +10V

–10V

76.29µV

±5V RANGE

+5V

–5V

38.15µV

+FS – (–FS)

218

LSB =

VIN

REF

2.5V

±5V CODE =

× 131,072 ×

VIN

10V

REF

2.5V

±10V CODE =

× 131,072 ×

Figure 36. AD7608 Transfer Characteristic

The LSB size is dependent on the analog input range selected.

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