1 Features

? Single-chip solution to directly measure 256 lowlevel

currents simultaneously

? Adjustable full-scale charge range up to 320 pC

? Input current: 1 μA (maximum)

? Adjustable speed with integration times as low as

50 μs (20 KSPS per channel)

? Resolution: 24 bit

? Low power dissipation: 1.2 mW/channel

? Integral non-linearity: ±0.025 of reading ±1 ppm

of full scale range (all channels active)

? Low noise: 0.26 fCrms at 320 pC FSR with 20 pF

sensor capacitance

? No charge loss

? On-chip temperature sensor

? Serial LVDS output interface

? Single 1.85-V supply

? In-package bypass capacitors and reference buffer

to reduce PCB area and design complexity

2 Applications

? CT Scanner data acquisition system

? Photodiode sensors

? X-ray detection systems

? Optical fiber power monitoring

? Muti-channel current, voltage instrumentation

3 Description

The DDC3256 is a 24-bit, 256-channel, current-input

analog-to-digital (A/D) converter. It combines both,

current-to-voltage conversion by current integration,

and A/D conversion.

Up to 256 individual low-level current output devices,

such as photodiodes, can be directly connected to its

inputs and digitized in parallel (simultaneously).

For each of the 256 inputs, the device has one

low noise and low power integrator designed to

capture all the charge from the sensor. The integration

time is adjustable from 50 μs to 1.6 ms, allowing

currents in the order of fA to μA to be continuously

measured with outstanding precision. The outputs of

the integrators are digitized by on-chip low power

ADCs and the converted digital codes are transmitted

over a single LVDS pair designed to minimize noise

coupling in environments with high channel count.

The DDC3256 operates from single 1.85-V supply.

The device is specified from 0°C to 70°C operating

temperature and available in a 13.2 × 17.2 mm2 336-

ball 0.8 mm-pitch BGA. The on-chip reference buffer

and bypass capacitors (on the BGA) help minimize

the external component requirements and further

reduce board space.