CN0548

Voltage and Current Measurement Connections

The CN0548 can be configured to accommodate a variety of measurement situations. The current sense input can be referenced to either the positive or negative voltage input terminal or any voltage in between, and measurement ground is isolated from the development platform and the connected host computer’s ground.

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

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

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

Figure 5 shows the connections for a high-side current measurement, with a supply voltage as high as 250 V. While the voltage measurement channel can tolerate voltages up to +/-250 V at its inputs without damage, the output will be saturated and will not produce a valid measurement.

Voltage Input

An LT1997-2 precision, high voltage funnel amplifier with internal matched resistor network scales the input voltage to within the input range of the ADC. This device has a gain error of 0.006% and a gain drift of 1 ppm/°C. Thirty eight unique attenuation factors can be selected by pin-strapping the +INA , +INB, +INC, -INA, -INB, and -INC inputs, implemented via jumpers on the CN0548.

Table 1 lists five jumper settings that cover most applications as well as the allowable input voltage for the circuit. 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 a live circuit, and do not move jumpers when connected to a live circuit.

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

Current Input

The AD8418A is a bidirectional, high voltage, zero-drift current sense amplifier. It features fixed gain of 20 V/V, with a 10 kHz bandwidth with a maximum ±0.15% gain error over its entire operating temperature range. The amplifier's output voltage directly interfaces to channel 1, AIN1- and AIN1+ of the ADC. The AD8418A offers excellent input common-mode rejection from -2 V to +70 V.

As shown in Table 3, the AD8418A performs bidirectional current measurements across a 10 mΩ, 2 W current sense resistor between the ISENSE input terminals. The maximum input current is+/-10 A in bipolar mode. The unipolar input range is 0 A to as high as 14 A, limited by the power dissipation of the sense resistor. The AD8418A output requires 32 mV of headroom to GND; refer to Unipolar and Bipolar Current Measurement Test Results.

Analog to Digital Conversion

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

The analog inputs to the AD7798 are fully differential, with an input range of 

The absolute voltage can extend to either supply rail when the inputs are in unbuffered mode and the in-amp is inactive, which is the configuration that the CN0548 uses by default.

The CN0548 presents high-level, buffered signals to the AD7798, so the gain can be set to 1 and buffers disabled, maximizing the input range. The 4.096 V reference voltage results in a +/-4.096 V input range, and readings are valid even when the ADC inputs are at or slightly below ground.

Reference Voltages

There are two voltage references used on the CN0548 board. An A-grade LT6657, shown in Figure 9, provides a 4.096 V reference voltage to the AD7798. This part offers exceptionally low noise for a bandgap reference; only 0.5 ppmP-P or an average of 1.24 μVp-p in the 0.1 Hz to 10 Hz bandwidth. It is stable with a large output capacitor, which serves to reduce high-frequency noise as well as providing a low-impedance for the AD7798’s dynamic sampling current. The line regulation of the LT6657 for 4.096 V output reference is typically well below 1 ppm/V. Load regulation is also less than 2 μV/mA. A 5 mA change in load current shifts output voltage by only 10 μV.

An LT6656 provides a 2.048 V bias voltage for the VSENSE amplifier, ISENSE amplifier, and ADC negative inputs, allowing the input terminals to accommodate bipolar ranges.

Noise Performance

The highest output noise voltage for the LT1997-2 is at attenuation of 4, which is about 1 μVp-p. The LT6657 has about 2 μVp-p output noise. The total (root-sum-square) value of 1 μVp-p and 2.0 μVp-p, or 1.7 μVp-p. The AD7798 quantization noise is 62.5μV, and will thus be the dominant noise source for voltage measurements. The input-referred noise will be approximately 1.2 mV in the 80 V input range.

For the current measurement mode, the AD8418A has an input noise voltage of 2.3 μVp-p from 0.1 Hz to 10 Hz. With a gain of 20, the reflected noise to the output results in 20 × 2.3 μVp-p, or 46 μVp-p. This is still slightly lower than the AD7798 quantization noise. While several codes of flicker are to be expected for a fixed, noiseless input, the AD7798 can still be considered the dominant noise source.

Power and SPI Isolation

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

Circuit Board Isolation

Figure 10 and Figure 11 show the circuit board isolation barrier. The board was designed for maximum creepage between grounds, with two 250 V safety rated type Y2 capacitors in series for reducing conducted noise from the LTM2886's internal switching regulator.