Feature: System Design
Improving System-level performance and robustness in power line monitoring
By Lluis Beltran Gil, applications engineer, Analog Devices
Background For many applications, monitoring power lines implies the use of current transformers and resistor divider networks in order to sense the three phases and neutral voltages and currents, as shown in Figure 1. The AD7606B, due to its high input impedance, can directly interface with a sensor, easing the data acquisition system design as AD7606B provides all the required building blocks. The AD7606B integrates, on-chip,
eight individual signal chains that accept either ±10V or ±5V true bipolar analogue input signals despite working from a single 5V supply. These features eliminate the need for driver op amps and external bipolar supplies. Each of these channels is comprised
of 21V analogue input clamp protection, a resistive programmable gain amplifier with 5MΩ input impedance, a first- order antialiasing filter, and a 16-bit SAR ADC. Also, an optional digital averaging filter with oversampling ratios of up to 256 and a low drift 2.5V
reference are included to help build a complete power line data acquisition system. In addition to the complete analogue signal chain provided, the AD7606B has plenty of calibration and diagnostic features to improve system-level performance and robustness.
Direct Sensor Interface Unlike AD7606, AD7606B input impedance has been increased to 5MΩ, which allows for it to directly interface with a wide variety of sensors while granting two straightforward benefits: • The gain error introduced by external series resistors (for example, the filtering or the resistor divider network) is reduced.
• The offset seen when the sensor is disconnected decreases, allowing for easy sensor disconnect detection features.
Gain Error Due to External Resistors In factory trimming, there is tight
22 November 2024
www.electronicsworld.co.uk control over RFB and RIN (5MΩ typical)
on a PGA, such that the AD7606B gain is accurately set. However, if an external resistor is placed in the front end, as shown in Figure 1, the actual gain then differs from the ideal trimmed RFB Te higher the RFILTER
/RIN , the greater the
gain error becomes, which will require compensation on the controller side. But the higher the RIN RFILTER
will have. Unlike the AD7606’s
1MΩ input impedance, the AD7606B has 5MΩ input, meaning that the gain error will reduce about 1 over 5 for the same series resistor (RFILTER
, the less effect the same .
) without any
calibration, as shown Figure 2. However, by using the AD7606B in
software mode, this system gain error can be automatically compensated on-chip, on a per channel basis, and completely eliminate the need for doing any gain calibration computation on the controller side.
Sensor Disconnect Detection Traditionally, having a pull-down resistor (RPD
) in parallel with the sensor
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