AD604ARS datasheet(16/20 Pages) AD

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AD604

REV. 0

–16–

Ultralow Noise, Differential Input-Differential Output VGA

Figure 48 shows how to use both preamplifiers and DSXs to

create a high impedance, differential input-differential output

variable gain amplifier. This application takes advantage of the

differential inputs to the DSXs. It should be pointed out that

the input is not truly differential, in the sense that the common-

mode voltage needs to be at ground to achieve maximum input

signal swing. This has mainly to do with the limited output

swing capability of the output drivers of the preamplifiers; they

clip around

±2.2 V due to having to drive an effective load of

about 30

Ω. If a different input common-mode voltage needs to

be accommodated, ac coupling (as was done in Figure 46) is

recommended. The differential gain range of this circuit runs

from +6 dB to +54 dB. This is 6 dB higher than each individual

channel of the AD604 because the DSX inputs now see twice

the signal amplitude compared to when they are driven single

ended.

AD604

0.1µF

C12

0.1µF

C13

0.1µF

+5V

–5V

ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.

0.1µF

453

Ω

+5V

–5V

+5V

0.1µF

453

Ω

0.1µF

VIN+

VIN–

VREF

VOUT+

VOUT–

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

Figure 48. Ultralow Noise, Differential Input–Differential

Output VGA

Figure 49 displays the output signals VOUT+ and VOUT– after

a –20 dB attenuator formed between the 453

Ω resistors shown

in Figure 48 and the 50

Ω loads presented by the oscilloscope

plug-in. R1 and R2 were inserted to insure a nominal load of

500

Ω at each output. The differential gain of the circuit was set

to +20 dB by applying a control voltage, VGN, of 1 V; the gain

scaling was 20 dB/V for a VREF of 2.500 V; the input frequency

was 10 MHz and the differential input amplitude 100 mV p-p.

The resulting differential output amplitude was 1 V p-p as can

be seen on the scope photo when reading the vertical scale as

200 mV/div.

100

20ns

20mV

+500mV

–500mV

ACTUAL

VOUT

NOTE 1. OUTPUT AFTER 10x ATTENUATER FORMED

BY 453

Ω TOGETHER WITH 50Ω OF 7A24 PLUG-IN.

Figure 49. Output of VGA in Figure 48 for VG = 1 V

Medical Ultrasound TGC Driving the AD9050, a 10-Bit,

40 MSPS A/D

The AD604 is an ideal candidate for the TGC (Time Gain

Control) amplifier that is required in medical ultrasound sys-

tems to limit the dynamic range of the signal that is presented to

the A/D converter. Figure 50 shows a schematic of an AD604

driving an AD9050 in a typical medical ultrasound application.

The gain is controlled by means of a digital byte that is input to

an AD7226 D/A converter that outputs the analog gain control

signal. The output common-mode voltage of the AD604 is set

to VPOS/2 by means of an internal voltage divider. The VOCM

pin is bypassed with a 0.1

µF to ground.

The DSX output is optionally filtered and then buffered by an

AD9631 op amp, a low distortion, low noise amplifier. The op

amp output is ac coupled into the self-biasing input of an

AD9050 A/D converter which is capable of outputting 10 bits at

a 40 MSPS sampling rate.

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