ANALOG DEVICES AD7400A Service Manual

Using the AD7400A Isolated ΣΔ Modulator as an

Isolated Amplifier

The AD74xx family of isolated ΣΔ ADCs is typically used in combination with an FPGA
or DSP, such as a Blackfin®, in motor drives to measure the phase-current across
shunts or monitor DC bus voltages. However, where a less capable microcontroller is
used, ΣΔ modulation may not be a suitable approach. That doesn’t mean the AD74xx
cannot be used in such situations, because there’s a straightforward way to convert the
AD74xx into an isolated amplifier using a simple RC filter. If you need to push the
performance envelope, you could also use more sophisticated active filters. This note
describes how to implement an isolated amplifier with an AD7400A.

Basic Operation of Isolated ΣΔ ADCs

The AD74xx ΣΔ ADCs convert analog input signals into a high-speed single-bit data­stream as shown in Figure 1. A differential signal of 0 V produces a stream of 1s and 0s
with a 50% duty cycle. The full-scale bipolar input range is ± 320mV with a resolution of
16 bits – in practice the usable linear input-range is ± 250mV for the AD7400A, which
translates into a bit-stream from 10.94% to 89.06% of 1s.

Figure 1 Analog Input vs. Modulator Output

How to Use AD7400A as an Isolated Amplifier

The modulated single-bit data-stream form the self-clocked AD7400A, with a typical
output data-rate of 10 MHz, resembles a pulse width modulated (PWM) signal. The
typical bandwidth of the generated output signal is only about 1 to 10 Hz. Through 2nd
order modulation of the AD7400A bit stream, the bandwidth can be much higher. A
simple RC filter has been shown to provide bandwidths up to 100 kHz. The block
diagram in Figure 2 shows the implementation with the ADuM5000 providing isolated
power to the secondary side. The ADuM5000 is a small form factor isolated DC/DC
converter.

© Copyright 2009 Analog Devices, Inc. 1

5V

V

V

DC/DC

ADuM5000

5

5V/3.3
CLK

DATA

Filter

Buffer

µC

ADuC702x

+/- 250mV

Figure 2 Block Diagram of AD7400A as Isolated Amplifier with ADuM5000 for Isolated Pow er

AD7400A

Considerations

There are some things to keep in mind when using the AD7400A as an isolated
amplifier. If the signal from the RC filter is used with a microcontroller, such as the
ADuC702x family of Analog Microcontrollers, it is recommended that an op amp be
used to match the input impedance of a SAR ADC. Calibration can also be performed,
enabling the microcontroller to remove errors in software. To achieve even better
performance, one could use an op amp as a buffer and configure the circuit with an
active buffer. The Analog Filter Wizard™ can be used to assist in such an
implementation (please refer to http://www.analog.com/en/amplifiers-and-comparators/products/dt-adisim-

design-sim-tool/Filter_Wizard/resources/fca.html).

Another consideration is that the standard digital output of the AD7400A does not have
guaranteed voltage levels – only minimum and maximum ranges are guaranteed. To
overcome this, one can use an analog switch, such as the ADG852, to alternate
between analog ground (AGND) and power (VDD) controlled by the output data. This
solution increases the flexibility of this design, allowing for larger output signal ranges,
such as ±15V.

This basic concept, using the AD7400A as an isolation amplifier, is interesting for a wide
range applications such as motor drives and patient monitoring, just to a name a few.
Please check out ADI’s “Circuits from the Lab” at www.analog.com/en/verifiedcircuits for updates
and more circuit ideas.

© Copyright 2009 Analog Devices, Inc. 2