AD8335 datasheet(23/24 Pages) AD

23 / 24 page

AD8335

Rev. 0 | Page 23 of 24

INPUT OVERDRIVE

Excellent overload behavior is of primary importance in ultra-

sound. Both the preamplifier and VGA have built-in overdrive

protection and quickly recover after an overload event.

Input Overload Protection

As with any amplifier, voltage clamping prior to the inputs is

highly recommended if the application is subject to high

transient voltages.

A block diagram of a simplified ultrasound transducer interface

is shown in Figure 59. A common transducer element serves the

dual functions of transmit and receive of ultrasound energy.

During the transmit phase, high voltage pulses are applied to

the ceramic elements. A typical T/R (transmit/receive) switch

may consist of four high voltage diodes in a bridge configuration.

Although they ideally block transmit pulses from the sensitive

receiver input, diode characteristics are not ideal, and resulting

leakage transients impinging on the PIPx inputs can be

problematic.

Since ultrasound is a pulse system, and time-of-flight is used to

determine depth, quick recovery from input overloads is essential.

Overload can occur in the preamp and the VGA. Immediately

following a transmit pulse, the typical VGA gains are low, and

the PrA is subject to overload from T/R switch leakage. With

increasing gain, the VGA can become overloaded from strong

echoes that occur with near field echoes and acoustically dense

materials, such as bone.

Figure 59 illustrates an external overload protection scheme. A

pair of back-to-back Schottky diodes is installed prior to installing

the ac-coupling capacitors. Although the BAS40 is shown, many

types are available and merit investigation by the user. With such

diodes, clamping levels of ±0.5 V or less greatly enhance the

system overload performance.

PrA

18dB

TRANSDUCER

–HV

BAS40-04

PMDx

PIPx

PONx

POPx

OPTIONAL

SCHOTTKY

OVERLOAD

CLAMP

RFB

+HV

Figure 59. Input Overload Protection

LOGIC INPUTS

The enable Pins EN12 and EN34, the preamp shutdown Pins SP12

and SP34, and the HILO Pins HL12 and HL34 are all logic inputs

of the AD8335. The enable inputs turn on and off each of the

corresponding pairs of channels; the preamp shutdown pins do

the same for the preamplifiers only; inputs HL12 and HL34 set

the HILO gain for Channels 1 and 2, and Channels 3 and 4,

respectively.

Shutting down the preamplifiers allows use of the VGAs alone,

while reducing power consumption. The VGAs cannot be shut

down independently. The SPxx (shutdown preamp) pins are

logic high; thus the pins are grounded to enable the preamplifiers.

The pins can be enabled by connecting to the supply or to

ground for fixed enable or disable, or to the output of a logic

device. Be sure to check the data sheet of the device for voltage

and current requirements.

COMMON-MODE PINS

The common-mode Pins VCMx are provided for bypassing the

internal common-mode reference for each channel to ground.

They require a capacitor at each of the four pins and can neither

be connected together nor driven by an external source.

DRIVING ADCs

The AD8335 VGA is designed to drive 10-bit and 12-bit ADCs

with minimal extra components. Because the AD8335 is a single

supply 5 V part and many of the newest ADCs operate from a

3 V supply, dissimilar common-mode voltages exist between the

VGA output and the ADC input. This level shift is most easily

accommodated by ac coupling, especially if the signal is filtered,

as is the case in most ultrasound and communications

applications.

When an anti-aliasing filter (AAF) is called for, it is advantageous

to implement a differential configuration. A fully differential

AAF requires approximately 1.5 times the number of components

than a single-ended filter, because the components that in the

single-ended case are tied to ground, now connect across the

differential signal path. Although the series components double,

the component count for the differential filter is more economical

when compared to simply building a pair of single-ended filters

requiring twice as many components.

AD8335 데이터시트(PDF) 23 Page - Analog Devices

AD8335 데이터시트(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

데이터시트 다운로드

링크 URL