Calibration
THE ON-CHIP CALIBRATION BENEFITS OF NEW SIMULTANEOUS SAR ANALOGUE-TO-DIGITAL CONVERTERS
This article, by Lluis Beltran Gil, product applications engineer at Analog Devices, evaluates the impact of external resistors used in front of resistive analogue-to-digital converters (ADCs). These families of simultaneous sampling ADCs include a high input impedance resistive programmable gain amplifier (PGA) that drives the ADC and scales the input signal, allowing direct sensor interface. However, there are several reasons why a design would end up adding external resistors in front of the analogue input. The following sections provide a theoretical explanation of the gain error expected, as a function of the resistor sizes, and different ways to minimise these errors. This article also examines resistor tolerance and the ADC’s input impedance effects of the different calibration options. Beyond the theoretical study, bench measurements compare several devices to prove the excellent accuracy reached with on-chip gain calibration features. The gain calibration feature enables a system error of less than 0.05 per cent, across a wide range of front-end resistor values, without having to perform any calibration routine, but just by writing a single register per channel.
S
imultaneous sampling successive approximation register (SAR) ADCs are traditionally defined as a response to the need expressed mainly by energy customers for protection relay applications.
In transmission and distribution networks, the protection relay monitors the grid to react to any fault condition (overvoltage or overcurrent) in the minimum time possible, to avoid critical damage. In order to monitor the power transmitted, both current and voltage need to be measured simultaneously. Current is measured through current transformers (CTs) that scale down the current, providing isolation and converting to voltages through a burden resistor. Voltage is measured through a resistor network, a voltage divider that scales the voltage down from the kV to V range. Analog Devices offers simultaneous sampling ADCs to monitor both voltages and current, simplifying the power calculations of dual, quad, or octal devices. Figure 1 shows a signal chain diagram normally used for measuring power in a single-phase, multi- phase electrical system, which would require a higher channel count data acquisition system (DAS) - that is, eight channels for three phases plus neutral.
WHEN TO USE EXTERNAL FRONT-END RESISTORS
Although the resistive input ADCs are designed to directly interface to most sensors, there are certain conditions where external resistors placed in front of the analogue inputs may be needed. This could be the case, for example, if an application calls for extra antialiasing filtering or protecting the inputs against overcurrent fault condition.
Antialiasing Filter
Even though the resistive input ADCs normally provide an internal antialiasing filter, many applications may run at lower sampling frequencies - hence, the need
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Figure 1. A typical signal chain in power monitoring applications. Only one phase is shown for simplicity. for lower corner frequency.
A common requirement is to gather 256 samples per power line cycle - that is, for a 50Hz power grid system a sampling frequency (fS) of 12.8kSPS.
overstress; however, it’s recommended to guard band the maximum limit by using a larger resistor, for example, 10kΩ.
Such low sampling frequency pushes the need for an external low-pass filter (LPF) in front of the resistive ADC’s inputs, suppressing any frequency above about 6.4kHz, the Nyquist frequency (fS/2). This can be achieved by adding a first-order RC filter.
Input Protection
In other application examples, especially within the protection relay market, there is a possibility of overcurrent flowing into the analogue input pins when a fault condition occurs. To avoid damaging the device, the absolute maximum rating (AMR) indicates to limit the input current under 10mA. To do so, placing an external in-series resistor is recommended, limiting such potential input current.
If the sensor output accidentally increases up to ±30V - as the input clamp protection circuitry can tolerate voltages up to ±16.5V - the input clamp protection circuit will turn on and sink a large current that can damage the device. Placing a 1.35kΩ RFILTER in front of the analogue inputs would prevent a current larger than 10mA to flow in during the
In any case, a larger resistor among the ones calculated from Equation 2 - that is, for either the antialiasing filter (AAF) or the current limit - must be used to ensure meeting both conditions. Note, however, that if the potential overstress of the analogue input signal sits below ±21V during a fault condition and there is no need for an external AAF, an external resistor may not be needed.
ERRORS INTRODUCED BY EXTERNAL RESISTORS
The drawback when introducing such external resistors - whether these are for extra filtering or protecting against a large current - is the impact they have on the system accuracy. The AD7606, for example, is factory trimmed to provide exceptionally low offset and gain errors - that is, 32 LSBs and 6 LSBs maximum respectively - across the full temperature and supply ranges. However, by adding external passives, these specifications are no longer valid, as the system gain error (understood by the system as the resistive input ADC plus the resistor in front) becomes larger
June 2026 Instrumentation Monthly
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