AD9631 datasheet(13/20 Pages) AD

13 / 20 page

AD9631/AD9632

REV. A

–13–

THEORY OF OPERATION

General

The AD9631 and AD9632 are wide bandwidth, voltage feed-

back amplifiers. Since their open-loop frequency response fol-

lows the conventional 6 dB/octave roll-off, their gain bandwidth

product is basically constant. Increasing their closed-loop gain

results in a corresponding decrease in small signal bandwidth.

This can be observed by noting the bandwidth specification

between the AD9631 (gain of 1) and AD9632 (gain of 2). The

AD9631/AD9632 typically maintain 65 degrees of phase mar-

gin. This high margin minimizes the effects of signal and noise

peaking.

Feedback Resistor Choice

The value of the feedback resistor is critical for optimum perfor-

mance on the AD9631 (gain +1) and less critical as the gain in-

creases. Therefore, this section is specifically targeted at the

AD9631.

At minimum stable gain (+1), the AD9631 provides optimum

dynamic performance with RF = 140 Ω. This resistor acts only

as a parasitic suppressor against damped RF oscillations that

can occur due to lead (input, feedback) inductance and parasitic

capacitance. This value of RF provides the best combination of

wide bandwidth, low parasitic peaking, and fast settling time.

In fact, for the same reasons, a 100–130

Ω resistor should be

placed in series with the positive input for other AD9631

noninverting and all AD9631 inverting configurations. The cor-

rect connection is shown in Figures 57 and 58.

–V

100–130

Ω

0.1

µF

0.1

µF

OUT

G = 1 +

AD9631/32

TERM

Figure 57. Noninverting Operation

–V

100–130

Ω

0.1

µF

0.1

µF

TERM

OUT

G = –

AD9631/32

Figure 58. Inverting Operation

When the AD9631 is used in the transimpedance (I to V) mode,

such as in photodiode detection, the value of RF and diode ca-

pacitance (CI) are usually known. Generally, the value of RF se-

lected will be in the k

Ω range, and a shunt capacitor (C

F) across

RF will be required to maintain good amplifier stability. The

value of CF required to maintain optimal flatness (<1 dB Peak-

ing) and settling time can be estimated as:

F ≅ (2 ωOCI RF –1)/ωO

[]1/2

where

O is equal to the unity gain bandwidth product of the

amplifier in rad/sec, and CI is the equivalent total input

capacitance at the inverting input. Typically

O = 800 × 10

rad/sec (see Open-Loop Frequency Response curve (Fig-

ure 17).

As an example, choosing RF = 10 kΩ and CI = 5 pF, requires

CF to be 1.1 pF (Note: CI includes both source and parasitic

circuit capacitance). The bandwidth of the amplifier can be es-

timated using the CF calculated as:

3 dB ≅

1.6

πR

F CF

OUT

AD9631

Figure 59. Transimpedance Configuration

AD9631 Arkusz danych(PDF) 13 Page - Analog Devices

AD9631 Arkusz danych(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Arkusz danych Pobierz

Link URL