SPONSORED FEATURE u Cover story
How to successfully calibrate an open-loop Dac signal cHain
by Martina Mincica, product applications engineer and alice o’Keeffe, analog devices Co-op Bipolar voltage DACs like the AD5766 can
A 8
ny practical electronic application is subject to multiple sources of error that can make the most precise components deviate from their data sheet behaviour. When the application
signal chain does not have a built-in mechanism to self-adjust for these errors, the only approach to minimise their impact is to measure them and calibrate them out systematically. An open-loop system is a system that does not
use its output to perform adjusting control actions on its inputs to achieve the required performance, while in a closed-loop system, the output depends on a control action in the system that can automatically implement corrections to improve performance. Most digital-to-analogue converter (DAC) signal chains are set-and-forget type systems, where the accuracy of the output must rely on the
accuracy of each block in the signal chain. A set-and- forget system is an open-loop system. For an open-loop system that requires high accuracy, calibration is most likely needed and recommended. Here, types of DAC signal chain calibration
will be introduced: the TempCal (calibration at operating temperature), which can give the best level of error correction, and the SpecCal (calibration using specifications), which is a valid alternative when using TempCal is not possible, but it is not as comprehensive.
Type of daCs A unipolar voltage DAC can only give output either of positive or of negative. This article will look at the AD5676R as an example of a unipolar DAC and how to accurately calibrate it. The same approach can be used to make necessary adjustments with other types of DACs.
Table 1. CalibraTion Types and errors ThaT Can be CorreCTed TempCal DAC Intrinsic Errors
Key Components Intrinsic Errors
Other System-Level Errors SpecCal
Can be included (for example, VREF)
x
achieve both positive and negative output. Current-output DACs are typically used in a
multiplying configuration (MDAC) to provide variable gain; they usually require external amplifiers to buffer the voltage generated across a fixed resistor. Precision current sourcing DACs (IDACs), like
the AD5770R and the LTC2662, are a new category of DACs that can precisely set an output current in a predefined range without requiring any additional external component.
daC Transfer funCTion Theory and inTrinsiC errors An ideal digital-to-analogue converter produces an analogue output voltage or current exactly proportional to the input digital code and independent of unwanted external influences like power supplies and reference variations. For an ideal voltage output DAC, the increase in
output for a single step increase in input digital code is called LSB and is defined as:
where: (VREF+) and (VREF-) are the positive and negative reference voltage. In some cases, (VREF-) is equal to ground (0 V). n is the resolution of the DAC in bits.
LSBSIZE (V) is the smallest increment to the DAC output in volts.
January 2022 Instrumentation Monthly
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