Calibration
Figure 11. On-chip calibration blocks. Only one channel is shown as an example.
This 8-bit register represents the RFILTER integer variable, allowing to compensate for an up
Figure 10. Back-end calibration error depending on the actual RIN value.
article. This on-chip gain calibration block knows exactly the input impedance (RIN), so it will always be more accurate than the back-end calibration, independent of the actual RIN and RFILTER values.
to 64kΩ resistor, with a resolution of 1024Ω.
Because of this discrete resolution, if the RFILTER is not a multiple of 1024, there will be a rounding error. The plot in Figure 12 shows how the post-calibration error stays below ±0.05%,
independent of RFILTER and RIN, provided both of them are used in calculating the calibration coefficient (K), given there is no assumption of
RIN being equal to its typical specification, but the actual internally measured RIN value is used instead. If compared to Figure 10, for the example
of an RFILTER = 30kΩ, this would mean up to 10× error reduction. Now the error is flat independent
of RFILTER, and the larger the RFILTER the greater the error reduction.
As the input impedance tolerance impacts the
calibration accuracy, the RFILTER tolerance will also impact the calibration accuracy. However, there are three points to note:
RFILTER is much smaller than RIN, plus discrete resistor tolerance would normally be better
than the internal 1MΩ input impedance tolerance.
The error introduced by the RFILTER tolerance will be present in both the back end and the
on-chip calibration schemes.
The user can minimise the RFILTER tolerance by using a lower tolerance discrete resistor.
Figure 12. On-chip calibration blocks, per channel.
A similar study can be performed, with the on-chip calibration feature enabled, assuming the RFILTER is
Figure 13. Impact of RFILTER discrete resistor toler- ance on the on-chip calibration feature accuracy
(worst-case scenario). TABLE 2. TOTAL ERROR (%) FOR DIFFERENT GENERICS, BOTH CALIBRATED AND UNCALIBRATED, FOR A GIVEN RFILTER AD7606B (5MΩ) RFILTER 10kΩ 20kΩ 50kΩ AD7606 Uncalibrated 0.5% 1.05% 2.5% *Worst-case error, independent of the RFILTER value Instrumentation Monthly June 2026 0.1% 0.2% 0.5% On-chip calibration* 0.01% 0.01% 0.01% Uncalibrated 0.45% 0.95% 2.5% On-chip calibration* 0.03% 0.03% 0.03% Continued on page 62... 61 AD7606C (1MΩ)
BENCH VERIFICATION Impact of the Input Impedance As expected from the previous theoretical analysis, the bench data in Figure 14 and Figure 15 show that
the five times higher input impedance (RIN) reduces approximately by five the impact the RFILTER resistor has on the system gain error. For example, a 20kΩ
resistor in front of the AD7606 (RIN = 1MΩ) would cause around 1% error, whereas this same resistor
in front of the AD7606B (RIN = 5MΩ) would cause around 0.2% error. However, a greater improvement
can be achieved by just turning on the on-chip gain calibration feature. There is no need to perform any
measurement; just write the RFILTER value, rounded to the nearest multiple of 1024Ω. By doing so, the error greatly reduces to less than 0.01%, as shown in Figure 14. Note that this error is effectively the total
instead at the worst case of its tolerance, for different common tolerances: 5%, 1%, and 0.1%.
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