®

VCA610

8

The VCA610’s gain control input has a –3dB bandwidth of

1MHz and varies with frequency as shown in the Typical

Performance Curves. This wide bandwidth, although useful

for many applications, can allow high frequency noise to

modulate the gain control input. In practice, this can be

easily avoided by filtering the control input as shown in

introduce gain errors by interacting with the gain control’s

input bias current of 2

µA.

INPUT PROTECTION

Electrostatic damage (ESD) has been well recognized for

MOSFET devices, but any semiconductor device deserves

protection from this potentially damaging source. The

VCA610 incorporates on-chip ESD protection diodes as

shown in Figure 4. This eliminates the need for the user to

add external protection diodes, which can add capacitance

and degrade AC performance.

ULTRASOUND TGC AMPLIFIER

The Figure 5 block diagram illustrates the fundamental

configuration common to pulse-echo imaging systems. A

piezoelectric crystal serves as both the ultrasonic pulse

generator and the echo monitor transducer. A transmit/

receive (T/R) switch isolates the monitor amplifier from the

crystal during the pulse generation cycle and, then, connects

the amplifier to the crystal during the echo monitor cycle.

FIGURE 4. Internal ESD Protection.

External

Pin

+V

S

–V

S

Internal

Circuitry

ESD Protection diodes internally

connected to all pins.

All pins on the VCA610 are internally protected from ESD

by means of a pair of back-to-back reverse-biased diodes to

either power supply as shown. These diodes will begin to

conduct when the pin voltage exceeds either power supply

by about 0.7V. This situation can occur with loss of the

amplifier’s power supplies while a signal source is still

present. The diodes can typically withstand a continuous

current of 30mA without destruction. To insure long term

reliability, however, diode current should be externally lim-

ited to 10mA whenever possible.

The internal protection diodes are designed to withstand

2.5kV (using Human Body Model) and provides adequate

ESD protection for most normal handling procedures. How-

ever, static protection is strongly recommended since static

damage can cause subtle changes in amplifier operational

characteristics without necessarily destroying the device.

APPLICATIONS

The electronically variable gain of the VCA610 suits pulse-

echo imaging systems well. Such applications include medical

imaging, non-destructive structural inspection and optical

distance measurement. The amplifier’s variable gain also

serves AGC amplifiers, amplitude-stabilized oscillators,

log amplifiers and exponential amplifiers. The discussions

below present examples of these applications.

FIGURE 5. Typical Ultrasound Application.

VCA610

V

C

Transducer

ADC

& DSP

V

C

t

0

–2V

T/R

Switch

Transmit

Receive

During the monitor (receive) cycle, the control voltage V

varies the amplifier gain. The gain is varied for three basic

signal processing requirements of a transducer array based

beamformer: compensation for depth attenuation effects,

sometimes called Time Gain Compensation (TGC); receive

apodization or windowing for reducing side lobe energy;

and dynamic aperture sizing for better near field resolution.

Time gain compensation increases the amplifier’s gain as

the ultrasound signal moves through the material to compen-

sate for signal attenuation versus material depth. For this

purpose, a ramp signal applied to the VCA610 gain control

input linearly increases the dB gain of the VCA610 with

time. The gain control provides signal apodization or

windowing with transducer arrays connected to amplifier

arrays. Selective weighting of amplifier gains across the

transducer aperture suppresses side lobe effects in the

beamformer output to reduce image artifacts. Gain con-

trolled attenuation or disabling the amplifier can be used to

dynamically size the array aperture for better near field

resolution. The controlled attenuation of the VCA610 mini-

mizes switching artifacts and eliminates the bright radial

rings that can result. The VCA610’s 80dB gain range ac-

commodates these functions.

WIDE-RANGE LOW-NOISE VCA

Figure 6 combines two VCA610s in series, extending the

overall gain range and improving noise performance. This

combination produces a gain equal to the sum of the two

amplifier’s logarithmic gains for a composite range of