AD604ARS datasheet(9/20 Pages) AD

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AD604

REV. 0

–9–

1To understand the active-feedback amplifier topology, refer to the AD830 data

sheet. The AD830 is a practical implementation of the idea.

THEORY OF OPERATION

The AD604 is a dual channel, variable gain amplifier with an

ultralow noise preamplifier. Figure 35 shows the simplified

block diagram of one channel. Each channel consists of:

(1) a preamplifier with gain setting resistors R5, R6 and R7

(2) a single-supply X-AMP (hereafter called, DSX, Differential

Single-supply X-AMP) made up of:

(a) a precision passive attenuator (differential ladder)

(b) a gain control block

(d) an Active Feedback Amplifier

1 (AFA) with gain setting

resistors R1 and R2

The preamplifier is powered by a

±5 V supply, while the DSX

uses a single +5 V supply. The linear-in-dB gain response of the

AD604 can generally be described by Equation 1:

G (dB) = (Gain Scaling (dB/V))

× (Gain Control (V )) +

((Preamp Gain (dB)) – 19 dB)

(1)

Each channel provides between 0 dB to +48.4 dB through +6 dB

to +54.4 dB of gain depending on the user determined pream-

plifier gain. The center 40 dB of gain is exactly linear-in-dB

while the gain error increases at the top and bottom of the

range. The gain of the preamplifier is typically either +14 dB or

+20 dB, but can be set to intermediate values by a single exter-

nal resistor (see PREAMPLIFIER section for details). The gain

of the DSX can vary from –14 dB to +34.4 dB which is deter-

mined by the gain control voltage (VGN). The VREF input

establishes the gain scaling – the useful gain scaling range is

between 20 dB/V and 40 dB/V for a VREF voltage of 2.5 V and

1.25 V respectively. For example, if the preamp gain was set to

+14 dB and VREF was set to 2.50 V (to establish a gain scaling

of 20 dB/V), the gain equation would simplify to:

G (dB ) = (20 (dB/V ))

× (VGN (V )) – 5 dB

The desired gain can then be achieved by setting the unipolar

gain control (VGN) to a voltage within its nominal operating

range of 0.25 V to 2.65 V (for 20 dB/V gain scaling). The gain is

monotonic for a complete gain control voltage range of 0.1 V to

2.9 V. Maximum gain can be achieved at a VGN of 2.9 V.

Since the two channels are identical, only Channel 1 will be

used to describe their operation. VREF and VOCM are the only

inputs that are shared by the two channels, and since they are

normally ac grounds, crosstalk between the two channels is

minimized. For highest gain scaling accuracy, VREF should

have an external low impedance voltage source. For low accu-

racy 20 dB/V applications, the VREF input can be decoupled

with a capacitor to ground. In this mode the gain scaling will be

determined by the midpoint between +VCC and GND, so care

should be taken to control the supply voltage to +5 V. The in-

put resistance looking into the VREF pin is 10 k

Ω ± 20%.

The DSX portion of the AD604 is a single-supply circuit and

the VOCM pin is used to establish the dc level of the midpoint

of this portion of the circuit. VOCM needs only an external

decoupling capacitor to ground to center the midpoint between

the supply voltages (+5 V, GND); however, if the dc level of the

output is important to the user (see APPLICATIONS section

for AD9050 example), then VOCM can be specifically set. The

input resistance looking into the VOCM pin is 45 k

Ω ± 20%.

Preamplifier

The input capability of the following single-supply DSX (2.5

2 V for a +5 V supply) limits the maximum input voltage of the

preamplifier to

±400 mV for the 14 dB gain configuration or

±200 mV for the 20 dB gain configuration.

The preamplifier’s gain can be programmed to +14 dB or

+20 dB; by either shorting the FBK1 node to PAO1 (+14 dB),

or leaving node FBK1 open (+20 dB). These two gain settings

are very accurate since they are set by the ratio of on-chip resis-

tors. Any intermediate gain can be achieved by connecting the

appropriate resistor value between PAO1 and FBK1 according

to Equations 2 and 3:

OUT

( R7 R

EXT ) + R5 + R6

(2)

REXT =

[R6

×G −(R5+ R6)]× R7

−(R6 ×G )+(R5+ R6)

(3)

FBK

820

Ω

VREF

VGN

PAI

PAO

+DSX

–DSX

EXT.

COM

Ω

VPOS

VOCM

200k

Ω

OUT

DIFFERENTIAL

ATTENUATOR

DISTRIBUTED GM

175

Ω

175

Ω

GAIN

CONTROL

Ω

200k

Ω

EXT.

Figure 35. Simplified Block Diagram of a Single Channel of the AD604

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