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Data Sheet

AD603

Rev. K | Page 13 of 24

SCALING

REFERENCE

GAIN-

CONTROL

INTERFACE

AD603

PRECISION PASSIVE

INPUT ATTENUATOR

FIXED-GAIN

AMPLIFIER

*NOMINAL VALUES.

R-2R LADDER NETWORK

VPOS

VNEG

GPOS

GNEG

VINP

COMM

0dB

–6.02dB –12.04dB –18.06dB –24.08dB –30.1dB –36.12dB –42.14dB

RRR

20Ω*

694Ω*

6.44kΩ*

VOUT

FDBK

Figure 31. Simplified Block Diagram

THE GAIN CONTROL INTERFACE

The attenuation is controlled through a differential, high

impedance (50 MΩ) input, with a scaling factor that is laser-

trimmed to 40 dB per volt, that is, 25 mV/dB. An internal band

gap reference ensures stability of the scaling with respect to

supply and temperature variations.

When the differential input voltage VG = 0 V, the attenuator

slider is centered, providing an attenuation of 21.07 dB. For the

maximum bandwidth range, this results in an overall gain of

10 dB (= −21.07 dB + 31.07 dB). When the control input is

−500 mV, the gain is lowered by +20 dB (= 0.500 V × 40 dB/V)

to −10 dB; when set to +500 mV, the gain is increased by

+20 dB to +30 dB. When this interface is overdriven in either

direction, the gain approaches either −11.07 dB (= − 42.14 dB +

+31.07 dB) or 31.07 dB (= 0 + 31.07 dB), respectively. The only

constraint on the gain control voltage is that it be kept within

the common-mode range (−1.2 V to +2.0 V assuming +5 V

supplies) of the gain control interface.

The basic gain of the AD603 can therefore be calculated by

Gain (dB) = 40 VG +10

(1)

where VG is in volts. When Pin 5 and Pin 7 are strapped (see the

Programming the Fixed-Gain Amplifier Using Pin Strapping

section), the gain becomes

Gain (dB) = 40 VG + 20 for 0 to +40 dB

and

Gain (dB) = 40 VG + 30 for +10 to +50 dB

(2)

The high impedance gain control input ensures minimal

loading when driving many amplifiers in multiple channel

or cascaded applications. The differential capability provides

flexibility in choosing the appropriate signal levels and

polarities for various control schemes.

For example, if the gain is to be controlled by a DAC providing

a positive-only, ground-referenced output, the gain control low

(GNEG) pin should be biased to a fixed offset of 500 mV to set

the gain to −10 dB when gain control high (GPOS) is at zero,

and to 30 dB when at 1.00 V.

It is a simple matter to include a voltage divider to achieve other

scaling factors. When using an 8-bit DAC having an FS output

of 2.55 V (10 mV/bit), a divider ratio of 2 (generating 5 mV/bit)

results in a gain-setting resolution of 0.2 dB/bit. The use of such

offsets is valuable when two AD603s are cascaded, when

various options exist for optimizing the signal-to-noise profile,

as is shown in the Sequential Mode (Optimal SNR) section,

PROGRAMMING THE FIXED-GAIN AMPLIFIER

USING PIN STRAPPING

Access to the feedback network is provided at Pin 5 (FDBK).

The user may program the gain of the output amplifier of the

AD603 using this pin, as shown in Figure 32, Figure 33, and

Figure 34. There are three modes: in the default mode, FDBK

is unconnected, providing the range +9 dB/+51 dB; when VOUT

and FDBK are shorted, the gain is lowered to −11 dB/+31 dB;

and, when an external resistor is placed between VOUT and

FDBK, any intermediate gain can be achieved, for example,

−1 dB/+41 dB. Figure 35 shows the nominal maximum gain vs.

external resistor for this mode.

GPOS

GNEG

VINP

COMM

VPOS

VOUT

VNEG

FDBK

AD603

VC1

VC2

VIN

VPOS

VOUT

VNEG

Figure 32. −10 dB to +30 dB; 90 MHz Bandwidth

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