AD7708 datasheet(17/44 Pages) AD

17 / 44 page

REV. 0

AD7708/AD7718

–17–

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 255

OUTPUT DATA RATE = 5.35Hz

INPUT BANDWIDTH = 1.28Hz

50Hz REJECTION = –93dB, 50Hz 1Hz REJECTION = –93dB

60Hz REJECTION = –74dB, 60Hz 1Hz REJECTION = –68dB

Figure 7. Filter Profile with SF = 255

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 69

OUTPUT DATA RATE = 19.8Hz

INPUT BANDWIDTH = 4.74Hz

FIRST NOTCH = 9.9Hz

50Hz REJECTION = –66dB, 50Hz 1Hz REJECTION = –60dB

60Hz REJECTION = –117dB, 60Hz 1Hz REJECTION = –94dB

Figure 8. Filter Profile with Default SF = 69 Giving Filter

Notches at Both 50 Hz and 60 Hz

FREQUENCY – Hz

–80

–160

100

–20

–40

–120

–140

–60

–100

SF = 68

OUTPUT DATA RATE = 20.07Hz

INPUT BANDWIDTH = 4.82Hz

50Hz REJECTION = –74dB, 50Hz 1Hz REJECTION = –54.6dB

60Hz REJECTION = –147dB, 60Hz 1Hz REJECTION = –101dB

Figure 9. Filter Profile with SF = 68

ADC NOISE PERFORMANCE CHOP ENABLED

(

CHOP = 0)

Tables II to V show the output rms noise and output peak-to-

peak resolution in bits (rounded to the nearest 0.5 LSB) for a

selection of output update rates. The numbers are typical and

generated at a differential input voltage of 0 V with AVDD =

DVDD = 5 V and using a 2.5 V reference. The output update

rate is selected via the SF7–SF0 bits in the Filter Register. It is

important to note that the peak-to-peak resolution figures repre-

sent the resolution for which there will be no code flicker within

a six-sigma limit. The output noise comes from two sources. The

first is the electrical noise in the semiconductor devices (device

noise) used in the implementation of the modulator. Secondly,

when the analog input is converted into the digital domain,

quantization noise is added. The device noise is at a low level

and is independent of frequency. The quantization noise starts at

an even lower level but rises rapidly with increasing frequency to

become the dominant noise source. The numbers in the tables

are given for the bipolar input ranges. For the unipolar ranges

the rms noise numbers will be the same as the bipolar range, but

the peak-to-peak resolution is now based on half the signal range

which effectively means losing one bit of resolution.

AD7708 Datasheet(PDF) 17 Page - Analog Devices

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