AD28MSP01 datasheet(16/28 Pages) AD

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AD28msp01

REV. A

–16–

DESIGN CONSIDERATIONS

Analog Input

The analog input signal to the AD28msp01 must be ac coupled.

Figure 12 shows the recommended input circuit for the

AD28msp01’s analog input pin (VIN). The circuit of Figure 12

implements a first-order low-pass filter with a 3 dB point at

20 kHz; this is the only filter that must be implemented external

to the AD28msp01 to prevent aliasing of the sampled signal.

Since the AD28msp01’s ADC uses a highly oversampled ap-

proach that transfers the bulk of the anti-aliasing filtering into

the digital domain, the off-chip anti-aliasing filter need only be

of low order.

In the circuit shown in Figure 12, scaling of the analog input is

achieved by the resistors RIN and RFB. The input signal gain,

–RFB/RIN, can be adjusted by varying the values of these resis-

tors. Total gain must be configured to ensure that a full-scale in-

put signal (at CIN in Figure 12) produces a signal level at the

input to the sigma-delta modulator of the ADC that does not

exceed VINMAX, which is specified under “Analog Interface Elec-

trical Characteristics.” If the total gain is increased above unity

(i.e., gain >1), signal-to-noise (SNR + THD) performance may

not meet the listed specifications.

The dc offsetting of the analog input signal is accomplished with

an on-chip voltage reference which nominally equals 2.5 V. The

input signal must be ac coupled with an external coupling ca-

pacitor (CIN). CIN and RIN should be chosen to ensure a cou-

pling corner frequency of 30 Hz. CIN should be 0.1 µF or larger.

VOLTAGE

REFERENCE

AD28msp01

CFB

CIN

INPUT

SIGNAL

RIN

RFB

VFB

VIN

Figure 12. Recommended Analog Input Circuit

To select values for the components shown in Figure 12, use the

following equations:

Gain

– R

IN =

π R

FB =

π)(20 *10

) R

10 k

Ω ≤ R

FB, RIN ≤ 50 kΩ

150 pF

≤ C

≤ 600 pF

Figure 13 shows an example of a typical input circuit configured

for 0 dB gain. The circuit’s diodes are used to prevent the input

signal from exceeding maximum limits.

INPUT

SIGNAL

10k

Ω

1.0

µF

10k

Ω

20k

Ω

330pF

GND

VOLTAGE

REFERENCE

AD28msp01

Figure 13. Typical Input Circuit (0 dB Gain)

Analog Output

The AD28msp01’s differential analog output (VOUTP, VOUTN) is

produced by an on-chip differential amplifier. The differential

amplifier can drive a minimum load of 2 k

Ω (R

L ≥ 2 kΩ) and

has a maximum differential output voltage swing of 6.312 V

peak-to-peak (3.17 dBm0). The differential output can be ac-

coupled directly to a load or dc-coupled to an external amplifier.

AD28msp01

OUTP

OUTN

OUT

Figure 14. Example Circuit for Differential Output with AC

Coupling

Figure 14 shows a simple circuit providing a differential output

with ac coupling. The capacitor of this circuit (COUT) is op-

tional; if used, its value can be chosen as follows:

OUT =

(60

π) R

The VOUTP–VOUTN outputs must be used as differential outputs;

do not use either as a single-ended output. Figure 15 shows an

example circuit which can he used to convert the differential

output to a single-ended output. The circuit uses a differential-

to-single-ended amplifier, the Analog Devices SSM2141.

AD28MSP01 Datasheet(PDF) 16 Page - Analog Devices

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