AD4884BBCZ datasheet(35/116 Pages) AD

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Data Sheet

AD4884

APPLICATIONS INFORMATION

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Rev. 0 | 35 of 116

ANALOG INPUTS DRIVE AND GAIN

Each AD4884 channel has an integrated fully-differential amplifier

stage at the input. As shown in Figure 58, the feedback resistors

are fixed at 750Ω, whereas a set of three resistors can be arranged

in different ways to change the value of the effective series input

resistance, and hence modify the gain of this stage (see Figure 61

to Figure 74). Gains beyond the 5.77 configuration result in a noise

contribution from the FDA stage that dominates over that of the

ADC itself, so they are generally not recommended.

Figure 58. AD4884 Input (Shown for Channel A)

Table 11 shows the gains that can be obtained through different

input pin short configurations, along with the resulting input impe-

dance for both differential and single-ended driving. Notation is

according to Figure 59 and Figure 60, and the differential gain is

calculated as G = RF/RG.

The input of the AD4884 can be driven with either a differential

or a single-ended signal. For the single-ended case, note that the

input impedance of the circuit is effectively higher than it is for a

conventional operation amplifier (op amp) connected as an inverter

because a fraction of the differential output voltage appears at the

inputs as a common-mode signal, partially bootstrapping the volt-

age across the RG input resistor. As for the common-mode of the

input signal, note that absolute voltage limits (Input Common-Mode

Voltage Range specification) at the FDA SJ± pins must be respect-

ed. In Table 11, the permitted common mode range is worked out

for each gain under nominal FDA supply conditions (+VS = 5V,

−VS = −1V), including some headroom to avoid violating the Input

Common-Mode Voltage Range specification even on a transient

basis. For more details on the common-mode at the output of the

FDA, see the Reference Buffer and Common-Mode Output section.

Figure 59. FDA Input Impedance, Differential Drive

Figure 60. FDA Input Impedance, Single-Ended Drive

Table 11. FDA Gain Options

Gain

Differential Input

Voltage Range (V)

Input Signal Common-Mode

Range (+VS = 5V, −VS = −1V)

Required External

Shorting

FDA Stage Input

RG (Ω)

RIN,DIFF

(Ω)

RIN,SE

(Ω)

1.03

±2.92

−3.2V to +5.6V

None

IN1±

360 + 240 + 130 = 730

1460

978

1.25

±2.40

−2.8V to +5.2V

IN3↔SJ

IN1±

360 + 240 = 600

1200

831

1.53

±1.96

−2.4V to +4.9V

IN2↔IN3

IN1±

360 + 130 = 490

980

702

2.03

±1.48

−2.0V to +4.6V

IN1↔IN2

IN1± or IN2±

240 + 130 = 370

740

556

2.08

±1.44

−2.0V to +4.6V

IN2↔IN3↔SJ

IN1±

360

720

544

2.74

±1.10

−1.7V to +4.3V

IN1↔IN3

IN2±

(360 || 240) + 130 = 274

548

432

3.13

±0.96

−1.6V to +4.2V

IN1↔IN2, IN3↔SJ

IN1± or IN2±

240

480

386

4.11

±0.73

−1.4V to +4.1V

IN1↔SJ

IN2±

360 || (240 + 130) = 182.5

365

305

5.21

±0.58

−1.3V to +4.0V

IN1↔IN3↔SJ

IN2±

(360 || 240) = 144

288

248

5.77

±0.52

−1.3V to +3.9V

IN1↔IN2↔IN3

IN1± or IN2± or IN3±

130

260

227

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