How to Quickly Build an Isolated High-Voltage, High-Current Measurement Module

How to Quickly Build an Isolated High-Voltage, High-Current Measurement Module

Testing and evaluating power systems in industrial and communications environments often requires multiple voltage and current measurements. Individual supplies may be referenced to different grounds, may have positive or negative polarity, or may be floating with no clear relationship to other power domains. Typically, these scenarios require the use of separate floating multimeters, or multi-channel meters with channels isolated from each other, but these meters are usually bulky and expensive.

In response to this, ADI has designed a simple and easy-to-use isolated current and voltage measurement system circuit (as shown in Figure 1), which can be used in industrial, telecommunications, instrumentation, and automated test equipment (ATE) applications. The system has electrical isolation characteristics, and the maximum allowable +/-250V between the main controller and the measurement ground is +/-250V. The isolation design contains digital data and power domain signals; no additional power is required from the circuit being measured.

Figure 1. CN0548 functional block diagram

The current input range is +/-10A, and the selectable voltage input range is 16V to 80V, with multiple ranges between these values ​​selectable. The voltage and current inputs feature 16-bit resolution, adjustable output data rate and signal bandwidth, including modes to reject 50Hz and 60Hz line noise.

The circuit is compatible with Arduino form factor platform boards and supports logic voltages from 1.8 V to 5 V. When paired with the open source firmware examples, application software can easily communicate with the reference design through the Linux Industrial Input/Output (IIO) framework using the libiio library, which includes bindings for C, C#, MATLAB, Python, and LabVIEW.

Evaluation and Design Support

►Circuit Evaluation Board

► CN-0548 Circuit Evaluation Board (EVAL-CN0548-ARDZ)

► ADuCM3029 Ultra Low Power Cortex-M3 Arduino Form Factor Development Board (EVAL-ADICUP3029)

► Design and integration documentation

►Schematics , layout files, bill of materials, mechanical drawings, software

Circuit Description

Voltage and current measurement connections

The CN0548 can be configured to support a variety of measurement situations. The current sense input can use the positive or negative voltage input terminal, or any voltage in between, as a reference source, and the measurement ground is isolated from the ground of the development platform and the connected host.

Figure 2 shows the connections for measuring a circuit with a grounded 15V supply and a grounded load. The load current is measured at the high side of the load.

Figure 2. +15V high-side current and voltage measurement

Figure 3 shows the connections for measuring the load current at the low side of the load (ground return).

Figure 3. +15V low-side current and voltage measurement

Figure 4 shows the connections for measuring a -48V supply, with the current measured in the ground return of the load.

Figure 4. -48V voltage and current measurement

Figure 5 shows the high-side current measurement connections with supply voltages up to 250 V. While the voltage measurement channels can tolerate voltages up to +/-250 V at the input without damage, the output will saturate and no valid measurement will be produced.

Figure 5. +250V system current measurement

Voltage input

The LT1997-2 precision high voltage funnel amplifier has internal matched resistor networks to adjust the input voltage to the input voltage range of the ADC. The device has a gain error of 0.006% and a gain drift of 1ppm/°C. 38 unique attenuation factors can be selected by pin-shorting the +INA, +INB, +INC, -INA, -INB, and -INC inputs, which are implemented via jumpers on the CN0548. Table 1 lists the five jumper settings that cover the allowable input voltages for most applications and circuits. Please refer to the LT1997-2 data sheet for a comprehensive list of attenuation factors. Note that the gain setting jumpers should be configured before connecting the CN0548 to live circuits and should not be moved while connected to live circuits.

Figure 6. Voltage range and polarity circuit

Table 1. Voltage Range Jumper Configuration

The CN0548 voltage input can be set to a unipolar or bipolar input range by configuring the LT1997-2 REF pin and the AD7798 AIN3 pin voltage as shown in Table 2.

Table 2. Unipolar/Bipolar Voltage Configurations

Current input

The AD8418A is a bidirectional, high voltage, zero-drift current sense amplifier. It has a fixed gain of 20V/V, a 10kHz bandwidth, and a maximum gain error of ±0.15% over the entire operating temperature range. The output voltage of the amplifier connects directly to Channel 1, AIN1-, and AIN1+ of the ADC. The AD8418A provides excellent input common-mode rejection from -2V to +70V. As shown in Table 3, the AD8418A performs bidirectional current measurements through a 10mΩ, 2W current sense resistor between the ISENSE input terminals. In bipolar mode, the maximum input current is +/-10A. The unipolar input range is 0A to up to 14A, limited by the power dissipation of the sense resistor. The AD8418A output requires 32mV of headroom to GND; see the unipolar and bipolar current measurement test results.

Figure 7. Current input signal conditioning and polarity circuit

Table 3. Unipolar/Bipolar Current Configuration

Analog-to-digital conversion

The AD7798 is a 16-bit, low power, high precision ∑-∆ analog-to-digital converter (ADC) designed to measure wide dynamic range, low frequency signals such as those found in pressure sensors, weigh scales, and precision measurement applications. The AD7798 has three buffered differential inputs with programmable instrumentation amplifiers and on-chip digital filtering. An external reference voltage of 100mV to 5.25V determines the full-scale input range. The output data rate of the AD7798 is user programmable from 4.17 to 470sps; the measurement bandwidth, as well as the noise sensitivity, is proportional to the output data rate. Most power measurement applications do not require high sampling rates and can take advantage of the narrow bandwidth provided by the lower output data rate modes. In addition, sampling rates of 16.7sps and lower can provide simultaneous rejection of 50Hz and 60Hz line noise. The AD7798 uses slightly different filter types depending on the output data rate to minimize the effects of internal noise sources. Figure 8 shows the filter response in 16.7Hz mode. Refer to the AD7798 data sheet for a complete and detailed description of all filter modes.

Figure 8. AD7798 filter response, 16.7Hz update rate mode

The analog inputs of the AD7798 are fully differential with an input range of . The absolute voltage can extend to either supply rail when the input voltage is in unbuffered mode and the instrumentation amplifier is idle, which is the default configuration used by the CN0548.

The CN0548 provides a high level buffered signal to the AD7798, so the gain can be set to 1 and the buffer disabled to maximize the input range. The 4.096V reference produces an input range of +/-4.096V, and the readings are valid even when the ADC input is at or slightly below ground.

Reference voltage

Two references are used on the CN0548 board. The A-grade LT6657 (shown in Figure 9) provides the 4.096V reference to the AD7798. This device offers very low noise for a bandgap reference; it provides only 0.5ppmP-P, or an average of 1.24μVp-p, over a 0.1Hz to 10Hz bandwidth. It is stable with a large output capacitor, which is used to reduce high frequency noise and provide a low impedance to the dynamic sampling current of the AD7798. The LT6657 typically provides less than 1ppm/V line regulation to the 4.096V output reference. Load regulation is also less than 2µV/mA. A 5mA change in load current shifts the output voltage by only 10µV.

Figure 9. 4.096V ADC reference voltage source

The LT6656 provides a 2.048V bias voltage for the VSENSE amplifier, ISENSE amplifier, and ADC negative input, enabling the input port to support a bipolar range.

Noise performance

The maximum output noise voltage of the LT1997-2 is attenuated by 4, which is about 1μVp-p. The LT6657 provides about 2μVp-p output noise. The total value (square root of the sum) is 1μVp-p and 2.0μVp-p, or 1.7μVp-p. The quantization noise of the AD7798 is 62.5μV, so it will be the dominant noise source in the voltage measurement. Over an 80V input range, the input referred noise is about 1.2mV.

In current measurement mode, the input noise voltage of the AD8418A is 2.3 μVp-p from 0.1 Hz to 10 Hz. With a gain of 20, the reflected noise voltage at the output is 20 × 2.3 μVp-p, or 46 μVp-p. This is still slightly lower than the quantization noise of the AD7798. Although a fixed, noise-free input may produce a few flicker codes, the AD7798 can still be considered the dominant noise source.

Power and SPI isolation

The 5V Serial Peripheral Interface (SPI) version of the LTM2886 µModule provides isolated +/-5V power and isolated SPI communications. No external components are required, and the decoupling capacitors are integrated into the module. The LTM2886 is extremely tolerant to common mode transients between ground planes; error-free operation is maintained through common mode events greater than 30kV/us. The LTM2886 includes an independent logic supply pin, allowing the logic level voltage on the host side to be any voltage between 1.62V and 5.5V.

Circuit board isolation

Figures 10 and 11 show the circuit board isolation barrier. The board is used to provide maximum creepage distance between grounds, and two safety-type Y2 capacitors rated for 250V are used in series to reduce conducted noise from the internal switching regulator of the LTM2886.