The circuit shown in Figure 1 is a complete low-cost analog/analog isolator solution that provides an isolation value of 2500 V rms (1 minute, per UL 1577).

This circuit is based on the AD7400A , a second-order Σ-Δ modulator that provides a digitally isolated 1-bit data stream output. The isolated analog signal is recovered using a fourth-order active filter based on the AD8646 dual-channel, low-noise, rail-to-rail op amp . The ADuM5000 is used as the power supply for the isolated side. Both ends are completely isolated and the system only uses one power supply. The circuit has 0.05% linearity and benefits from the noise shaping provided by the AD7400A modulator and analog filters. Applications for this circuit include motor control and current monitoring, and it can effectively replace opto-isolator-based isolation systems.

Figure 1 shows the block diagram of the circuit. The analog input is sampled by the AD7400A Σ-Δ modulator at 10 MSPS. The 22 Ω resistor and 0.1 μF capacitor form a differential input noise reduction filter with a cutoff frequency of 145 kHz. The output of the AD7400A is an isolated 1-bit data stream. The quantization noise is shaped by a second-order Σ-Δ modulator, which moves the noise to higher frequencies (see tutorial MT-022 ).

To reconstruct the analog input signal, the data stream should be followed by an ADG849 switch and connected to a 3 V ADR443 reference to stabilize the peak-to-peak output of the MDAT.

The signal is then filtered through an active filter with a higher order than the modulator order. For better noise attenuation, a fourth-order Chebyshev filter is used. When the filter order is the same, the Chebyshev response provides the steepest roll-off compared to other filter responses (Butterworth or Bessel). The filter is implemented using the AD8646 dual-channel, rail-to-rail input and output, low-noise, single-supply op amp.

The ADuM5000 is an isolated DC/DC converter based on ADI's iCoupler® technology, used to provide power to the isolated side of the circuit (including ^the AD7400A ) . isoPower® technology utilizes high-frequency switching elements to transfer power through chip-scale transformers.

This circuit must be built on a multilayer circuit board with a large area ground plane. For optimal performance, proper layout, grounding, and decoupling techniques must be used (refer to Tutorial MT-031 - "Grounding Data Converters and Solving the Mysteries of AGND and DGND" , Tutorial MT-101 - " decoupling technology" and ADuC7060/ADuC7061) evaluation board layout. Special care should be taken when designing printed circuit board (PCB) layout, which must comply with relevant radiation standards and isolation requirements between two isolated terminals. (See application note AN-0971 .)

To avoid overdriving the AD8646, the input signal should be below the supply voltage of the AD8646 (5 V). The output of the AD7400A is a data stream of ones and zeros with an amplitude equal to the AD7400A V _DD2 supply voltage. Therefore, the V _DD2 digital supply is the 3.3 V voltage provided by the linear regulator ADP121 . Alternatively, if a 5 V supply is used for VDD2, the digital output signal should be attenuated before being connected to the active filter. In either case, the power supply should be properly regulated as the final analog output is directly proportional to VDD2.

The 5 V supply for the circuit shown in Figure 1 is provided by the ADP3301 5 V linear regulator , which accepts input voltages from 5.5 V to 12 V.

Analog Active Filter Design

The cutoff frequency of a low-pass filter mainly depends on the required bandwidth of the circuit. There is a trade-off between cutoff frequency and noise performance; if you increase the cutoff frequency of the filter, the noise will increase. This is especially true in this design because the Σ-Δ modulator shapes the noise, moving a large portion to higher frequencies. This design chooses a cutoff frequency of 100kHz.

For a given cutoff frequency, the smaller the transition band of the filter, the less noise the filter passes. Of all filter responses (Butterworth, Chebyshev, Bessel, etc.), the Chebyshev response was chosen for this design because it has a smaller transition band for a given filter order, but The trade-off is slightly worse transient response performance.

This filter is a fourth-order filter consisting of two second-order filters using a Sallen-Key structure. The filter was designed using the Analog Filter Wizard and Ni Multisim tools. All parameters include:

• Filter type = low pass, Chebyshev, 0.01dB ripple
• Order = 4
• F _c = 100 kHz, Sallen-Key (updated format for clarity)

All program-generated recommended values ​​are used except that the feedback resistor is reduced to 22Ω.

Measurement

The AD7400A has a gain of 5.15 and an output offset voltage of 1.65 V (when powered from a 3.3 V supply). A differential signal of 0 V produces a digital bit stream of 1s and 0s, with 1s and 0s each occupying 50% of the time. The digital output supply is 3.3V, so there is a 1.65 V DC offset after filtering. In ideal conditions, a 320 mV differential input generates a data stream of all ones, which is filtered to produce a 3.3 V DC output. Therefore, the effective gain of the AD7400A is:

Gain = (3.3 − 1.65)/0.32 = 5.15625

By measurement, the measured offset is 1.641497 V and the gain is 5.165. The DC transfer function of the system is shown in Figure 2. The measured linearity is 0.0465%.

Figure 3 shows the output voltage versus input frequency without DC offset voltage. The input signal voltage is 40 mV pp, which produces an output signal of 40 × 5.165 = 207 mV pp. Note that there is about 10 mV peaking in the frequency response function, which is equivalent to about 0.42 dB.

The system has good noise performance, with noise densities of 2.50 μV/√Hz at 1 kHz and 1.52 μV/√Hz at 10 kHz.

For the complete design support package for this circuit note, see http://www.analog.com/CN0185-DesignSupport .