AD600ARZ-R7 datasheet(16/28 Pages) AD

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AD600/AD602

Rev. E | Page 16 of 28

REALIZING OTHER GAIN RANGES

Larger gain ranges can be accommodated by cascading

amplifiers. Combinations built by cascading two amplifiers

include −20 dB to +60 dB (using one AD602), −10 dB to +70 dB

(using ½ of an AD602 followed by ½ of an AD600), and 0 dB to

80 dB (using one AD600). In multiple-channel applications,

extra protection against oscillation can be provided by using

amplifier sections from different packages.

AN ULTRALOW NOISE VCA

The two channels of the AD600 or AD602 can operate in

parallel to achieve a 3 dB improvement in noise level, providing

1 nV/√Hz without any loss of gain accuracy or bandwidth.

In the simplest case, as shown in Figure 35, the signal inputs

A1HI and A2HI are tied directly together. The outputs A1OP

and A2OP are summed via R1 and R2 (100 Ω each), and the

control inputs C1HI/C2HI and C1LO/C2LO operate in parallel.

Using these connections, both the input and output resistances

are 50 Ω. Thus, when driven from a 50 Ω source and terminated

in a 50 Ω load, the gain is reduced by 12 dB, so the gain range

becomes –12 dB to +28 dB for the AD600 and −22 dB to

+18 dB for the AD602. The peak input capability remains

unaffected (1 V rms at the IC pins, or 2 V rms from an

unloaded 50 Ω source). The loading on each output, with a

50 Ω load, is effectively 200 Ω, because the load current is

shared between the two channels, so the overall amplifier still

meets its specified maximum output and distortion levels for a

200 Ω load. This amplifier can deliver a maximum sine wave

power of 10 dBm to the load.

VPOS

VNEG

100Ω

50Ω

GAIN-CONTROL

VOLTAGE

–+

VIN

VOUT

REF

AD600 OR

AD602

–

C1HI

A1CM

A1OP

A2OP

A2CM

C2HI

C1LO

A1HI

A1LO

GAT1

A2LO

A2HI

C2LO

GAT2

+5V

–5V

Figure 35. An Ultralow Noise VCA Using the AD600 or AD602

A LOW NOISE, 6 dB PREAMPLIFIER

In some ultrasound applications, a high input impedance

preamplifier is needed to avoid the signal attenuation that

results from loading the transducer by the 100 Ω input resistance

of the X-AMP. High gain cannot be tolerated because the

peak transducer signal is typically ±0.5 V, while the peak input

capability of the AD600 or AD602 is only slightly more than

±1 V. A gain of 2 is a suitable choice. It can be shown that if the

preamplifier’s overall referred-to-input (RTI) noise is the same

as that due to the X-AMP alone (1.4 nV/√Hz), the input noise

of nX2 preamplifier must be √(3/4) times as large, that is,

1.2 nV/√Hz.

+5V

–5V

+5V

–5V

1µF

0.1µF

VIN

INPUT

GROUND

OUTPUT

GROUND

49.9Ω

174Ω

42.2Ω

562Ω

174Ω

49.9Ω

MRF904

MM4049

100Ω

RIN OF X-AMP

562Ω

1µF

42.2Ω

1µF

Figure 36. A Low Noise Preamplifier for the AD600/AD602

An inexpensive circuit using complementary transistor types

chosen for their low rbb is shown in Figure 36. The gain is

determined by the ratio of the net collector load resistance to

the net emitter resistance. It is an open-loop amplifier. The gain

is ×2 (6 dB) only into a 100 Ω load, assumed to be provided by

the input resistance of the X-AMP; R2 and R7 are in shunt

with this load, and their value is important in defining the gain.

For small-signal inputs, both transistors contribute an equal

transconductance that is rendered less sensitive to signal level

by the emitter resistors, R4 and R5. They also play a dominant

role in setting the gain.

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