128-Channel Digital X-Ray Analog Front End

The AD8488 is a 128-channel analog front end 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, CSA and CSB select the lower and upper 64-blocks of 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.

Applications

* High performance digital X-ray systems

* Medical X-ray

* Security (baggage scanner) systems

* 128 integrator channels

* Correlated double sample error correction (CDS)

- Corrects for VOS and LF noise

* Power consumption/channel

- Normal: 11 mW

- Low power: 4 mW

* Low input leakage current

- 1.5 pA typical

* Low input referred noise (QNI)

- 1000 e−rms typ

* Linearity error

- 0.03% typical

* Compact 17 mm × 17 mm BGA

* Selectable filter time constants

* 4 selectable input charge ranges

* 10 selectable gain ranges