AD73422_15 datasheet(11/36 Pages) AD

Part #	AD73422

Description	Dual Low Power CMOS Analog Front End with DSP Microcomputer

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REV. 0

AD73422

–11–

Table I. PGA Settings for the Encoder Channel

IGS2

IGS1

IGS0

Gain (dB)

000

0

001

6

010

12

011

18

100

20

101

26

110

32

111

38

ADC

Both ADCs consist of an analog sigma-delta modulator and a

digital antialiasing decimation filter. The sigma-delta modu-

lator noise-shapes the signal and produces 1-bit samples at a

DMCLK/8 rate. This bitstream, representing the analog input

signal, is input to the antialiasing decimation filter. The decima-

tion filter reduces the sample rate and increases the resolution.

Analog Sigma-Delta Modulator

The AD73422’s input channels employ a sigma-delta conver-

sion technique, which provides a high resolution 16-bit output

with system filtering being implemented on-chip.

Sigma-delta converters employ a technique known as over-

sampling where the sampling rate is many times the highest

frequency of interest. In the case of the AD73422, the initial

sampling rate of the sigma-delta modulator is DMCLK/8. The

main effect of oversampling is that the quantization noise is

spread over a very wide bandwidth, up to FS/2 = DMCLK/16

(Figure 4a). This means that the noise in the band of interest is

much reduced. Another complementary feature of sigma-delta

converters is the use of a technique called noise-shaping. This

technique has the effect of pushing the noise from the band of

interest to an out-of-band position (Figure 4b). The combination

BAND

OF

INTEREST

FS/2

DMCLK/16

a.

FS/2

DMCLK/16

NOISE-SHAPING

b.

BAND

OF

INTEREST

FS/2

DMCLK/16

DIGITAL FILTER

BAND

OF

INTEREST

c.

Figure 4. Sigma-Delta Noise Reduction

of these techniques, followed by the application of a digital

filter, sufficiently reduces the noise in band to ensure good

dynamic performance from the part (Figure 4c).

Figure 5 shows the various stages of filtering that are employed

in a typical AD73422 application. In Figure 5a we see the trans-

fer function of the external analog antialias filter. Even though it

is a single RC pole, its cutoff frequency is sufficiently far away

from the initial sampling frequency (DMCLK/8) that it takes

care of any signals that could be aliased by the sampling fre-

quency. This also shows the major difference between the initial

oversampling rate and the bandwidth of interest. In Figure 5b,

the signal and noise-shaping responses of the sigma-delta modu-

lator are shown. The signal response provides further rejection

of any high frequency signals, while the noise-shaping will push

the inherent quantization noise to an out-of-band position. The

detail of Figure 5c shows the response of the digital decima-

tion filter (Sinc-cubed response) with nulls every multiple of

DMCLK/256, which corresponds to the decimation filter up-

date rate for a 64 kHz sampling. The nulls of the Sinc3 response

correspond with multiples of the chosen sampling frequency.

The final detail in Figure 5d shows the application of a final

antialias filter in the DSP engine. This has the advantage of

being implemented according to the user’s requirements and

available MIPS. The filtering in Figures 5a through 5c is imple-

mented in the AD73422.

FB = 4kHz

FSINIT = DMCLK/8

a. Analog Antialias Filter Transfer Function

FB = 4kHz

FSINIT = DMCLK/8

NOISE TRANSFER FUNCTION

SIGNAL TRANSFER FUNCTION

b. Analog Sigma-Delta Modulator Transfer Function

FB = 4kHz

FSINTER = DMCLK/256

c. Digital Decimator Transfer Function

FB = 4kHz

FSINTER = DMCLK/256

FSFINAL = 8kHz

d. Final Filter LPF (HPF) Transfer Function

Figure 5. ADC Frequency Responses

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