AD28msp02

REV. 0

–2–

OUTPUT

DIFFERENTIAL

AMP

CONTROL

REGISTER

VOLTAGE

REFERENCE

1

16

16

INPUT

AMP

ANALOG

SIGMA-DELTA

MODULATOR

1.0

MHz

ANTI-ALIASING

DECIMATION

FILTER

DIGITAL

HIGH-PASS

FILTER

8.0

kHz

8.0

kHz

16-BIT SIGMA-DELTA ADC

16

8.0

kHz

SDI

SDIFS

SDO

SDOFS

SCLK

DATA/

CNTRL

MUX

INPUT

AMP

VIN

AUX

+20dB

AMP

NORM

AUX

CS

1

16

16

1.0

MHz

1.0

MHz

8.0

kHz

16-BIT SIGMA-DELTA DAC

ANTI-IMAGING

INTERPOLATION

FILTER

PGA

SERIAL

PORT

HIGH-PASS

DIGITAL

FILTER

ANALOG

SMOOTHING

FILTER

DIGITAL

SIGMA-DELTA

MODULATOR

Figure 1. AD28msp02 Block Diagram

The IIR low-pass filter is a 10th-order elliptic filter with a pass-

band edge at 3.7 kHz and a stopband attenuation of 65 dB at

4 kHz. This filter has the following specifications:

Filter type:

10th-order low-pass elliptic IIR

Sample frequency:

40.0 kHz

Passband cutoff:*

3.70 kHz

Passband ripple:

±0.2 dB

Stopband cutoff:

4.0 kHz

Stopband ripple:

–65.00 dB

*The passband cutoff frequency is defined to be the last point in the passband

that meets the passband ripple specification.

(Note that these specifications apply only to this filter, and not to the entire

ADC. The specifications can be used to perform further analysis of the exact

characteristics of the filter, for example using a digital filter design software

package.)

Figure 2 shows the frequency response of the IIR low-pass filter.

0

–100

5000

–80

2000

–40

–60

–20

4400

3800

3200

2600

FREQUENCY – Hz

Figure 2. IIR Low-Pass Filter Frequency Response

High-Pass Filter

The digital high-pass filter removes frequency components at

the low end of the spectrum; it attenuates signal energy below

the passband of the converter. The high-pass filter can be

bypassed by setting the ADBY bit (Bit 3) of the AD28msp02’s

control register.

The high-pass filter is a 4th-order elliptic filter with a passband

cutoff at 150 Hz. Stopband attenuation is 25 dB. This filter has

the following specifications:

Filter type:

4th-order high-pass elliptic IIR

Sample frequency:

8.0 kHz

Passband cutoff:

150.0 Hz

Passband ripple:

±0.2 dB

Stopband cutoff:

100.0 Hz

Stopband ripple:

–25.00 dB

(Note that these specifications apply only to this filter, and not to the entire

ADC. The specifications can be used to perform further analysis of the exact

characteristics of the filter, for example using a digital filter design software

package.)

Figure 3 shows the frequency response of the high-pass filter.

0

–100

300

–80

0

–40

–60

–20

240

180

120

60

FREQUENCY – Hz

Figure 3. High-Pass Filter Frequency Response

Passband ripple is

±0.2 dB for the combined effects of the

ADC’s digital filters (i.e., high-pass filter and IIR low-pass of

the decimation filter) in the 300 Hz–3400 Hz passband.

The output of the ADC is transferred to the AD28msp02’s

serial port (SPORT) at an 8 kHz rate, for transmission to the

host DSP processor. Maximum group delay in the ADC will not

exceed 1 ms in the region from 300 Hz to 3 kHz.