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Description
Summary
The digitization of the signals in the CMS Electromagnetic Calorimeter and in the
Preshower detectors require a data converter with a demanding combination of wide
dynamic range, high speed, good resolution and low power consumption. To avoid the
requirements for a very high precision radiation hard ADC the approach has been to
use multiple gain ranges to span the overall dynamic range, digitizing and
transmitting the signals of only the highest unsaturated range. Thus a 12 bit ADC is
sufficient. This approach leads clearly to a simpler system, as the range choice is
performed digitally by the converter. The ADC must also be capable to withstand the
extreme radiation environment of the LHC experiments. To fulfill these requirements
a custom made ADC has been designed and fabricated using a commercial 0.25μm CMOS
technology. In order to evaluate the production characteristics of the newly
fabricated AD41240 ADC component a special characterization testbench has been
developed. The testbench consists of a custom made hardware setup capable to host
the device under test and apply the necessary test conditions. Software in Matlab
has been developed to analyze the converted data and compute the performance
characteristics. Tests were performed under many different conditions to evaluate
the behavior under worst case conditions.
For the production testing of the ADCs we have chosen to develop a similar setup as
the one used for the device characterization. An easy to use graphical user
interface has been developed in Visual Basic in order to automatize the operation of
the testbench. Extensive characterization is economically unacceptable in high
volume production testing. Therefore a subset of the characterization tests has been
chosen that can guarantee that no bad components are shipped. Despite the relatively
short test time allocated for this job, a full sample of 512Msamples has been
collected for each device and the characteristics of each chip have been extracted
by computing the static parameters (INL, DNL) as weel as the dynamic parameters
(SNR, SFDR, THD and ENOB). In addition to this, a standard set of DC parameters
(Idd, Vbg, Vcm) have been collected allowing to compare each device to a reference
target device.
This paper describes the design architecture of the AD41240 A/D converter,
introduces the characterization methodology that has been employed and presents the
production test bench that has been developed and operated as well as the test
procedures that have been employed.
Devices have finally been selected by applying fairly selective cuts on the
distribution of static and dynamic parameters, still allowing us to pass more than
80% of the production chips into real utilization.
