AD871JD 查看數據表（PDF） - Analog Devices

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AD871JD

Complete 12-Bit 5 MSPS Monolithic A/D Converter

Analog Devices 

AD871

Figure 12 shows the common-mode rejection performance vs.

frequency for a 1 V p-p common-mode input. This excellent

common-mode rejection over a wide bandwidth affords the user

the opportunity to eliminate many potential sources of input

noise as common mode by using the differential input structure

of the AD871.

–40

–50

–60

–70

–80

–90

–100

10k

100k

1M

10M

INPUT FREQUENCY – Hz

Figure 12. Common-Mode Rejection vs. Input Frequency,

1 V p-p Input

Figures 13 and 14 illustrate typical input connections for single

ended inputs.

؎1V

1 VINA

AD871

2 VINB

Figure 13. AD871 Single-Ended Input Connection

؎1V

1 VINA

RT

AD871

2 VINB

Figure 14. AD871 Single-Ended Input Connection Using a

Shielded Cable

The cable shield is used as the ground connection for the VINB

input, providing the best possible rejection of the cable noise

from the input signal. Note also that the high input impedance

of the AD871 allows the user to select the termination imped-

ance, be it 50 ohms, 75 ohms, or some other value. Further-

more, unlike many flash converters, most AD871 applications

will not require an external buffer amplifier. If such an amplifier

is required, we suggest either the AD811 or AD9617.

Figure 15 illustrates how external amplifiers may be used to

convert a single-ended input into a differential signal. The resis-

tor values of 536 Ω and 562 Ω were selected to provide opti-

mum phase matching between U1 and U2.

562⍀

VIN

(؎0.5V)

536⍀

562⍀

U1

536⍀

U2

VINA

AD871

VINB

Figure 15. Single-Ended to Differential Connections;

U1, U2 = AD811 or AD9617

The use of the differential input signal can help to minimize

even-order distortion from the input THA where performance

beyond –70 dB is desired.

Figure 16 shows the AD871 large signal (–0.5 dB) and small

signal (–20 dB) frequency response.

10

0

–10

–20

–30

104

105

106

107

108

INPUT FREQUENCY – Hz

Figure 16. Full Power (–0.5 dB) and Small Signal

Response (–20 dB) vs. Input Frequency

The AD871’s wide input bandwidth facilitates rapid acquisition

of transient input signals: the input THA can typically settle to

12-bit accuracy from a full-scale input step in less than 80 ns. Fig-

ure 17 illustrates the typical acquisition of a full-scale input step.

4400

4000

3600

3200

2800

2400

2000

1600

1200

800

400

0

0

20

40

60

80

100

TIME – ns

Figure 17. Typical AD871 Settling Time

–10–

REV. A

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