128-Channel

Digital X-Ray Analog Front End

Data Sheet

AD8488

Rev. A

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FEATURES

128 integrator channels

Correlated double sample error correction (CDS)

Corrects for VOS and LF noise

Power consumption per channel

Normal: 11 mW

Low power: 4 mW

Low input leakage current: −1.5 pA typical

Low input referred noise (QNI): 993 e−rms typical

Linearity error: 0.03% typical

Compact 17 mm × 17 mm BGA

Selectable filter time constants

4 selectable input charge ranges

10 selectable gain ranges

APPLICATION

High performance digital X-ray systems

Medical X-ray

Security (baggage scanner) systems

GENERAL DESCRIPTION

The AD8488 is a 128-channel, analog front end (AFE) designed for

use in high performance digital X-ray systems. The analog channels

consist of an integrator followed by a gain selectable single-ended

to low impedance differential output. The analog channel converts

the charge acquired by X-ray or photodiode detectors to a voltage.

The channels are composed of CMOS transistors, using typical

high input impedance CMOS gates. The integrators generate charge

dependent voltages using a range of selectable capacitance values that

accommodate a broad range of input charge values. The integrators

are followed by single-ended input to differential output voltage

amplifiers where offset and low frequency noise voltages are

subtracted from the input voltages. A 128:1 channel differential

MUX follows the buffers and drives the analog output buffer.

Switch drivers and certain digital timing functions are included, and

all are mounted on a 255-lead BGA substrate. Charge conversion

for all 128 channels is simultaneous followed by a sequential voltage

output read of the channels using a 7-bit address code. The sequence

occurs twice, sampling all 128 channels. Logic control inputs, CS_A

and CS_B, select the lower and upper 64 blocks of the channel

addresses.

The AD8488 is packaged in a 17 mm × 17 mm, 255-lead, RoHS-

compliant ball grid array (BGA). The operating temperature range

is 0°C to 85°C ambient.

FUNCTIONAL BLOCK DIAGRAM

CHARGE CONVERSION AMPLIFIER

128 CHANNELS

LPF

INT

IN0:127

VREF

TIMING SIGNALS

ANALOG

INPUTS

GAIN AMPLIFIER

CONTROL REGISTER

MUX SELECTOR

TIMING AND CONTROL

CONTROL SIGNALS

2

4

2

7

DIFFERENTIAL

OUTPUT

MULTIPLEXER

128:1

OUTPUT

BUFFER

OUTHI

OUTLO

128

7

INT

Figure 1.