This circuit provides galvanic isolation for high-speed, high-precision, simultaneous sampling analog-to-digital conversion applications, as shown in Figure 1. The 16-bit PulSAR ADC AD7685 is a versatile device that supports monitoring of multiple channels through a daisy chain. The input circuit based on the AD8615 operational amplifier level-converts, attenuates, and buffers the ±10 V industrial signal to meet the input requirements of the ADC. This flexible circuit includes an ADR391 precision voltage reference and two ADuM1402 quad-channel digital isolators to provide a compact and cost-effective solution for common industrial data acquisition applications.

The AD7685 is a 16-bit, charge redistribution, successive approximation (SAR) analog-to-digital converter (ADC) that operates from a single 2.3 V to 5.5 V supply (VDD). It houses a low-power, high-speed, 16-bit no-missing-code, no-pipeline-delay sampling ADC, an internal conversion clock, and a multi-function serial interface port. It also integrates a low-noise, wide-bandwidth, short-aperture-delay sampling ADC. Maintain the circuit. On the rising edge of CNV, it samples the analog input voltage difference between IN+ and IN-, ranging from 0 V to VREF. The reference voltage VREF is provided externally and ranges from 0.5 V to VDD. The circuit shown in Figure 1 uses a 4.5 V reference voltage.

The power consumption of the AD7685 is linearly proportional to the sampling rate. At VDD = 5 V and a sampling rate of 250 kSPS, the maximum power consumption is 15 mW. The AD7685 is available in a 10-lead MSOP or 10-lead QFN (LFCSP) package and operates over the −40°C to +85°C temperature range. The SPI-compatible serial interface can also use the SDI input to daisy-chain several ADCs onto a single three-wire bus, or provide an optional busy (BUSY) indicator function. It uses independent VIO power pins and is compatible with 1.8V, 2.5V, 3V or 5V logic.

The complete analog signal chain operates from a single 5 V supply. A low dropout 2.5 V reference ADR391 and a U13 rail-to-rail CMOS op amp AD8615 generate the 4.5 V reference used by the ADC. The 4.5 V reference voltage provides 0.5 V headroom at the output of U13, so the op amp remains operating in the linear region over the nominal variation of the 5 V supply. The ADR391 2.5 V output is amplified by the noise gain of U13 (1 + R4/R5). For the chosen values ​​of R4 and R5, the noise gain is 1.8, so the reference voltage is 1.8 × 2.5 V = 4.5 V.

The AD8615 op amp U4 and U14 provides a signal gain of 0.225 (set by the ratio of R1 to R2 and R19 to R20), reducing the input signal amplitude of 20 V pp to 4.5 V pp at the ADC input. For a 0 V input, the outputs of U4 and U14 require a 2.25 V offset, which requires a common-mode voltage of 1.84 V at the non-inverting inputs of U4 and U14, which is generated by the resistor divider R3-R6.

The RC network (33 Ω, 2.7 nF) at the outputs of U4 and U14 forms a single-pole noise filter with a bandwidth of 1.8 MHz.

The AD8615 is a CMOS rail-to-rail input and output, single-supply amplifier that features very low offset voltage, wide signal bandwidth, and low input voltage and current noise. The device uses DigiTrim® patented trim technology to achieve outstanding accuracy without the need for laser trimming. The AD8615 operates from a single 2.7 V to 5 V supply.

The combination of 20+ MHz bandwidth, low offset, low noise, and low input bias current make this amplifier suitable for a variety of applications. Devices such as filters, integrators, photodiode amplifiers, and high-impedance sensors can benefit from this combination of features. The wide bandwidth and low distortion characteristics are beneficial for AC applications. In the DigiTrim family, the AD8615 provides the highest output drive capability and is ideal for audio line drivers and other low impedance applications.

Specific applications for this device include portable and low-power instrumentation, audio amplification for portable devices, portable telephone headsets, bar code scanners, and multi-pole filters. It also has rail-to-rail input and output swing capabilities, allowing designers to buffer CMOS ADCs, DACs, ASICs and other wide output swing devices in single-supply systems.

The ADR391 is a micropower, low dropout voltage reference that provides a stable output voltage from a supply that is only 300 mV above the output voltage. The advanced design eliminates the need for external capacitors, further saving board space and reducing costs. The ADR391 precision voltage reference features low power consumption, small size, and ease of use, making it ideal for battery-powered applications. The device utilizes Analog Devices' patented temperature drift curvature correction technology to achieve low temperature drift characteristics of 9 ppm/°C in a TSOT package.

The ADuM1402 is a four-channel digital isolator using Analog Devices' iCoupler® technology . The isolation device combines high-speed CMOS with single-chip air-core transformer technology to deliver superior performance characteristics than alternative devices such as optocouplers.

iCoupler devices do not use LEDs and photodiodes, so there are no design difficulties typically associated with optocouplers. The simple iCoupler digital interface and stable performance characteristics eliminate the uncertain current transfer ratios, nonlinear transfer functions, and temperature and lifetime effects typically associated with optocouplers.

These iCoupler products do not require external drivers and other discrete components. In addition, when the signal data rate is equivalent, the power consumption of iCoupler devices is only 1/10 to 1/6 of the optocoupler.

The ADuM1402 isolator provides four independent channels of isolation, supporting 2/2 direction and multiple data rates up to 90 Mbps (Grade C) (refer to the Ordering Guide section of the data sheet). All models operate from 2.7 V to 5.5 V supply voltages, are compatible with low-voltage systems, and are capable of voltage translation across the isolation barrier. In addition, the ADuM1402 features low pulse width distortion (less than 2 ns) and tight channel-to-channel matching (less than 2 ns). Unlike other optocouplers, the ADuM1402 isolator has a patented refresh feature that ensures dc correctness when there are no input logic transitions and when one supply is missing.

Figure 1 shows how the AD7685 can be daisy-chained to reduce the number of signals that need to be isolated. Note that RSCLK and RFS are the readback results of the AD7685 serial clock (SCK) and serial frame synchronization (CNV) respectively. The skew of these readback signals relative to the DATA signal must be very short. This skew is the channel-to-channel matching propagation delay (tPSKCD) of the digital isolator, which is less than 2 ns for the ADuM1402C. Therefore, the serial interface is capable of operating at the maximum speed of the digital isolator (90 Mbps for the ADuM1402C), which corresponds to a maximum serial clock frequency of 90 MHz. If the delay is too long, it may be limited by the digital isolator propagation delay cascade. In this circuit, for a sampling frequency of 250 kSPS, the TSCLK frequency is 30 MHz.

Apply two ±10 V signals with a phase difference of 90° to the two channels (AIN1 and AIN2) of the EVAL-CN0194-SDPZ board, and use the included evaluation software to obtain the conversion results, as shown in Figure 2. Use the included evaluation software to also view FFT data (Figure 3), as well as a code histogram at fixed DC levels (Figure 4).

The performance of this or any high-speed circuit is highly dependent on proper PCB layout, including but not limited to power supply bypassing, controlled impedance lines (if required), component placement, signal routing, and power and ground planes. (For details on PCB layout, see tutorials MT-031 , MT-101 , and the article A Practical Guide to High-Speed ​​Printed Circuit Board Layout .)

For the complete design support package for this circuit note, see www.analog.com/en/CN0194-DesignSupport .