Agilent Technologies DC122 User Manual download pdf (Page 19)

User Manual: Agilent Acqiris 10-bit Digitizers Page 19 of 27

acquisition is complete and the data is waiting to be readout. The user can override the default functions and program

the LED color in an application-specific manner.

3.5. External Clock and Reference

For applications where the user wants to replace the internal clock of the digitizer and drive the ADC with an

external source, either an External Clock or an External Reference signal can be used. The Clock or Reference

signals can be entered into the digitizer by the dedicated MMCX. In addition, the PXI Bus 10 MHz system clock

signal (PXI_CLK10) can be used as the reference.

The External Clock must be continuously present for use by these digitizers. The input signal must have a frequency

between 200 MHz and 2 GHz and a minimum amplitude of at least 0.5 V peak to peak into 50 Ω at the front of the

digitizer. The transitions of the clock are defined with the aid of a threshold that is user selectable in the range [-3.0

V, 3.0 V]. The signals should not exceed ±5 V amplitude. For a detailed discussion on the programmed use of the

external clock, refer to the Programmer’s Guide.

For applications that require greater timing precision and long-term stability than is obtainable from the internal

clock, a 10 MHz Reference signal can be used. The External Reference is nominally at 10 MHz. However,

frequencies in the range [9.97 MHz, 10.03 MHz] will be accepted. If you do this you may need to correct for the

difference in your application since the digitizer and the driver have no way to know about such deviations. The

amplitude and threshold conditions, for an External Reference, are the same as for the External Clock. If

synchronization between several digitizers is required, the reference signal should be applied to all of them.

3.6. Internal Calibration

The software drivers supplied include calibration functions for the timing, gain and offset settings, which can be

executed upon user request. The digitizers are never calibrated in an “automatic” way, i.e. as a side effect of another

operation. This ensures programmers have full control of all calibrations performed through software in order to

maintain proper event synchronization within automated test applications.

The digitizers include a high precision voltage source and a 16-bit DAC, used to determine the input voltage and

offset calibration.

For accurate time and voltage measurements it is recommended to perform a calibration once the module has attained

a stable operating temperature (usually reached within 15 minutes of digitizer operation after power on). Further

calibration should not be necessary unless temperature variations occur.

A full internal calibration of a digitizer can be very time consuming (> 100 s/digitizer), in particular for the HZ

models. Therefore, several other options are available. They are documented in the Programmer's Reference

Manual. A program can always be started with the digitizer in an uncalibrated state and data taken can be used for

many kinds of testing. However, as soon as good data respecting the specifications of the instrument is required a

calibration of at least the current acquisition state is needed. The full internal calibration has the advantage that it

generates the calibration constants needed for any possible configuration of the instrument; its disadvantage is the

time taken. If a more selective calibration is done it will allow the generation of good data in the current acquisition

state. This calibration will remain useable whenever that acquisition state is used again for as long as the temperature

of the instrument does not change significantly. A fast calibration of a channel in a configuration can be done in

around a second. Many applications can save time by only performing calibration for the configurations that will

actually be used. Calibration can usually be performed with signals present at the channel, external, and clock inputs.

However, if the calibration is found to be unreliable, as shown by a calibration failure status, it may be necessary to

remove such signals.

An application program will close devices if it exits normally. When a 10-bit digitizer is closed in this way its power

consumption will be reduced. This has the consequence that if another application program is started later an

appropriate warm-up period may have to be respected before a calibration is called for and full performance

expected.

3.7. Factory Calibration

Whilst the Internal Calibration function will provide a good degree of confidence that your instrument is operating

within its specifications on a day-to-day basis, Agilent recommends that each instrument undergoes a Factory

Calibration at least annually to ensure that it remains within the specified performances.

Factory Calibration is the process of measuring the actual performance of an instrument-under-test using lab

instruments that in turn have significantly better performance than the instrument-under-test. Lab instrument

performance must be traceable to the International System (SI) Units via a national metrology institute (NIST, NPL,

NRC, PTB, CENAM, INMETRO, BIPM, etc.)

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