CN0240

This circuit, shown in Figure 1, monitors bidirectional current from sources with dc voltages of up to ±270 V with less than 1% linearity error. The load current passes through a shunt resistor, which is external to the circuit. The shunt resistor value is chosen so that the shunt voltage is approximately 100 mV at maximum load current.

The AD629 amplifier accurately measures and buffers (G = 1) a small differential input voltage and rejects large positive common-mode voltages up to 270 V.

The dual AD8622 is used to amplify the output of the AD629 by a factor of 100. The AD8475 funnel amplifier attenuates the signal (G = 0.4), converts it from single-ended to differential, and level shifts the signal to satisfy the analog input voltage range of the AD7170 sigma-delta ADC.

Galvanic isolation is provided by the ADuM5402 quad channel isolator. This is not only for protection but also to isolate the downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital isolator can supply isolated +5.0 V for the circuit.

The measurement result from the AD7170 is provided as a digital code utilizing a simple 2-wire, SPI-compatible serial interface.

This combination of parts provides an accurate high voltage positive and negative rail current sense solution with a small component count, low cost, and low power.

Figure 1. High Common-Mode Voltage Bidirectional Isolated Current Monitor (All Connections and Decoupling Not Shown)

The circuit is designed for a full-scale shunt voltage of 100 mV at maximum load current IMAX. Therefore, the value of the shunt resistor is RSHUNT = (500 mV)/(IMAX).

The AD629, shown in Figure 2, is a difference amplifier designed with internal thin film resistors allowing continuous common-mode signals up to ±270 V with transient protection to ±500 V. For REF(+) and REF(−) grounded, the signal on the +IN terminal is attenuated by a factor of 20. The signal is then amplified by a noise gain of 20, restoring the original amplitude at the output.

Figure 2. AD629 High Common-Mode Voltage Difference Amplifier

The CMRR is 77 dB minimum @ 500 Hz for the AD629A, and 86 dB minimum @ 500 Hz for the AD629B.

In order to maintain the desired common-mode rejection, there are several important conditions to meet. First, the ability of the part to reject these common-mode signals is determined by the power supply voltage as shown in Figure 3. Failure to implement dual supplies of a sufficient voltage will reduce the common-mode rejection.

Figure 3. AD629 Common-Mode Voltage Range vs. Power Supply Voltage

Secondly, the AD629 should only be operated in the unity gain mode using the internal matched thin film resistors. Changing the gain with external resistors will degrade the common-mode rejection due to mismatch errors.

The AD8622 is a CMOS low power, precision, dual, rail-to-rail output op amp used primarily for amplifying the signal of interest.

By cascading two inverting gain stages with a gain of –10, the 100 mV full-scale output of the AD629 is amplified by a factor of 100 yielding a 10 V full-scale signal. These values can be either positive or negative, depending on the direction of the current.

The dual supplies of the AD8622 allow both the input and output signals to swing above and below ground as required to measure bidirectional input currents.

In the final stage of the signal chain before conversion into a digital word, the AD8622 output voltage is conditioned to fit the analog input voltage range of the ADC.

The AD8475 "funnel amplifier," shown in Figure 4, provides two optional attenuation factors (0.4 and 0.8). In addition, the signal is converted into a differential one, and the common-mode voltage at the output is determined by the voltage on the VOCM pin. With a single 5 V supply, the analog input voltage range is ±12.5 V (for a single-ended input).

Figure 4. AD8475 Funnel Amplifier

As shown in Figure 1, the output common-mode voltage is set at 2.5 V by a resistor divider driven by the ADR435 reference output of 5 V.

The primary source of noise in the system is the output noise of the AD629 of 15 μV p-p in the 0.1 Hz to 10 Hz bandwidth. For a 100 mV full-scale signal, this corresponds to a noise-free code resolution of

The input noise of the AD8622 is only 0.2 μV p-p, which is negligible compared to the AD629. The output noise of the AD8475 is 2.5 μV p-p, which is also negligible at that point where the full-scale signal level is 4 V p-p.

Notice that the power supply voltage for the AD7170 is supplied by the isolated power output (+5.0 VISO) of the ADuM5402 quad isolator.

The reference voltage for the AD7170 is supplied by the ADR435 precision XFET® reference. The ADR435 has an initial accuracy of ±0.12% (A grade), and a typical temperature coefficient of 2 ppm/°C. The ADR435 has a wide operating range (7.0 V to 18.0 V) and utilizes the +15.0 V rail for a power source.

Although it is possible to operate both the AD7170 VDD and REFIN(+) from the 5.0 V power supply, using a separate reference provides better accuracy.

The input voltage to the AD7170 ADC is converted into an offset binary code at the output of the ADC. The ADuM5402 provides the isolation for the DOUT data output, the SCLK input, and the PDRST input. Although the isolator is optional, it is recommended to protect the downstream digital circuitry from the high common-mode voltage in the case of a fault condition.

The code is processed in the PC by using the SDP hardware board and LabVIEW software.

Figure 5 shows a comparison between the code seen at the output of the ADC recorded by LabVIEW and an ideal code calculated based on a perfect system. The plots show how the circuit achieves an end-point linearity error of less than 0.5% over the entire input voltage range (−100 mV to +100 mV). The offset error and gain error can be removed using software calibration if desired.

Figure 5. Plot of Actual Code, Ideal Code, and % Error vs. Shunt Voltage

PCB Layout Considerations

There are a number of solutions available for high-side sensing of positive and negative sources. IC solutions using current sense amplifiers, difference amplifiers, or a combination of these are available. See the circuits described in the following circuit notes: CN0100, CN0188, CN0218.

“High-Side Current Sensing: Difference Amplifier vs. Current Sense Amplifier,” Analog Dialogue, January 2008, describes the use of current sense and difference amplifiers. The article is available at www.analog.com/HighSide_CurrentSensing.

The following URLs link to Analog Devices products useful in solving the current sense problem:

Current sense amplifiers: www.analog.com/CurrentSenseAmps
Difference amplifiers: www.analog.com/DifferenceAmps
Instrumentation amplifiers: www.analog.com/InstrumentationAmps

WARNING! HIGH VOLTAGE. THIS CIRCUIT MAY CONTAIN LETHAL VOLTAGES. DO NOT OPERATE, EVALUATE, OR TEST THIS CIRCUIT, OR BOARD ASSEMBLY, UNLESS YOU ARE A TRAINED PROFESSIONAL, WHO IS QUALIFIED TO HANDLE HIGH VOLTAGE CIRCUITRY. BEFORE APPLYING POWER, YOU MUST BE FAMILIAR WITH THE CIRCUITRY AND ALL REQUIRED PRECAUTIONS FOR WORKING WITH HIGH VOLTAGE CIRCUITS.

This circuit uses the EVAL-CN0240-SDPZ circuit board and the EVAL-SDP-CB1Z System Demonstration Platform (SDP) evaluation board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance. The EVAL-CN0240-SDPZ board contains the circuit to be evaluated, as described in this note, and the SDP evaluation board is used with the CN0240 evaluation software to capture the data from the EVAL-CN0240-SDPZ circuit board.


Equipment Needed

See Figure 1 of this circuit note for the circuit block diagram and the EVAL-CN0240-SDPZ-SCH pdf file for the circuit schematics. This file is contained in the CN0240 Design Support Package.


Setup