The circuit shown in Figure 1 is a cost-effective, highly integrated 16-bit, 250 kSPS, 8-channel data acquisition system that can digitize ±10 V industrial-grade signals. The circuit also provides 2500 V rms isolation between the measurement circuit and the host controller, and the entire circuit operates from a single isolated PWM-controlled 5 V supply.

This circuit is used with the 16-bit, 8-channel, 250 kSPS PulSAR ADC AD7689 and two low-cost precision quad AD8608 operational amplifiers to provide all signal conditioning and digitization functions within the data acquisition system. In addition, only the AD8605 operational amplifier is needed to buffer the reference voltage of the AD7689.

The AD8605 and AD8608 are low-cost single and quad rail-to-rail input and output CMOS amplifiers, respectively. The AD8608 inverts, level shifts, and attenuates ±10 V input signals to match the input range of the ADC, which is 0 V to +4.096 V when using a +4.096 V reference and a single +5 V supply. .

The AD8605 is used as an external reference voltage buffer to provide sufficient drive capability for level translation. The AD8605 and AD8608 have extremely low offset voltage, low input voltage and current noise, and wide signal bandwidth, making them suitable for a variety of applications. The AD8608's low current and voltage noise ensures that resistor noise is the dominant contributor to high input impedance output noise. The input impedance in this circuit (equal to R1) is 50 kΩ.

The AD7689 16-bit, 8-channel, 250 kSPS PulSAR ADC contains all the components required for a multi-channel, low-power data acquisition system. It includes a 16-bit SAR ADC, an 8-channel low crosstalk multiplexer, a low-drift reference and buffer, a temperature sensor, an optional single-pole filter, and a channel sequencer. The sequencer can be used to scan channels continuously and does not require a microcontroller or FPGA to control channel switching. The AD7689 is packaged in a small 20-pin, 4 mm × 4 mm LFCSP, minimizing cost and printed circuit board (PCB) area. Operating temperature range is −40°C to +85°C. Power consumption is 12.5 mW (typ) at 250 kSPS from a 5 V supply.

The ADuM3471 is a four-channel digital isolator that integrates a PWM controller and transformer driver to drive an isolated DC/DC converter. The ADuM3471 provides 5 V, 2 W isolated power to the circuit and isolates digital signals at the SPI interface.

Analog front-end design

In process control and industrial automation systems, typical signal levels are up to ±10 V. The circuit in Figure 1 uses an inverting amplifier with attenuation and level shifting to convert the ±10 V signal into a signal suitable for the ADC range.

The circuit formula is as follows:

The front-end signal gain (−R2/R1) is set to −0.2 so that the signal range reaching the ADC is 4 V peak-to-peak. This is consistent with an input range of 0 V to 4.096 V (equal to the reference voltage V _{REF ).}

For an OV input signal, the output of the op amp should be at midscale or 0.5 V _REF .

Substituting formula 1 into formula 2, we get

The common-mode voltage at the input of the op amp is calculated by:

For R3/R4 = 1.4 and V _REF = 4.096 V, the common-mode voltage of the op amp is 1.7 V.

There are four amplifiers inside each AD8608, and the four non-inverting inputs are shorted together and connected to the resistor divider R3/R4. The second voltage divider is for the second AD8608. To eliminate op amp input bias current,

The input impedance of the circuit is R1, which should ideally be higher. However, resistor thermal noise is proportional to the square root of the resistance, so system noise performance decreases as the resistor value increases. To determine the optimal value, a simple analysis of the noise is required.

According to the Nyquist criterion, the maximum signal frequency content should be less than half the maximum sampling rate. The AD7689 250 kSPS sampling rate produces a Nyquist frequency of 125 kHz. To minimize signal attenuation within this bandwidth, the front-end's −3 dB cutoff frequency is designed to be approximately 12 times the Nyquist frequency, or 1.5 MHz.

The noise model for this circuit is shown in Figure 2. There are three noise sources in this circuit: resistor noise, amplifier voltage noise, and amplifier current noise. The rms value for each noise source is shown in Table 1. For more information on op amp noise, see application note AN-358 and tutorials MT-047 , MT-048 , and MT-049 .

The total rms noise before the ADC should be less than 0.5 LSB within the target bandwidth so that the ADC can correctly digitize the input signal.

Resistor noise can be calculated by:

The unit of R is Ω.

The noise performance using the resistor values ​​shown in Figure 1 and a 1.5 MHz bandwidth is summarized in Table 1.

These uncorrelated noise voltages add in root-sum-square form; therefore, the total op amp output rms noise within a 1.5 MHz bandwidth is approximately 21.3 μV. For a 4.096 V reference, the 16-bit LSB is 62.5 μV. The rms noise of 21.3 μV is less than 0.5 LSB, so the resistor values ​​shown in Figure 1 are suitable for this application.

Note that the largest source of total output noise is resistor R2, which in this circuit is 10 kΩ. Reducing the value of R2 requires R1 to decrease proportionally, thus lowering the input impedance.

The input current noise of the AD8608 is very small and will not be a significant factor unless extremely large resistor values ​​are used. The low input current noise and input bias current of the AD8605 and AD8608 make them ideal amplifiers for high impedance sensors such as photodiodes.

Add capacitor C1 in parallel with R2 to form a single-pole, active low-pass filter. Bandwidth is calculated using Equation 7. Assuming a 1.5 MHz, −3 dB bandwidth, C1 is approximately 10 pF. In this circuit, considering the parasitic effects of the PCB board, a value of 8.2 pF is selected.

Analog-to-digital converter (ADC)

The AD7689 is a modern SAR ADC using an internal switched capacitor DAC. Due to the SAR architecture, there is no pipeline delay during the conversion process, greatly simplifying multiplexing operations. Figure 3 shows the equivalent analog input circuit. A small transient current is injected into the analog input at the sampling frequency, and an external filter network consisting of R5 and C2 reduces its effect on the op amp output. In addition, the filter bandwidth is 2.7 MHz, which reduces noise at the ADC input.

With selectable reference voltages of 4.096 V or 2.5 V, the input range of this circuit can be switched between ±10 V and ±6 V without degrading system resolution.

An internal temperature sensor can be used to monitor the AD7689's junction temperature to enable system calibration and temperature compensation in precision applications.

Single-chip solution for isolated power and digital I/O

The ADuM3471 is a single-chip solution for power and digital I/O isolation. Isolation voltage is 2500 V rms (UL 1577 device approved). The ADuM3471 provides 4 channels of isolated I/O ports and integrates a PWM controller and transformer driver for an isolated DC/DC converter. When used with some external components, the ADuM3471 can provide 2 W of isolated power from any regulated voltage (3 V to 24 V). The necessary external components are a transformer for power transfer, two Schottky diodes for full-wave rectification, an LC filter for ripple suppression and two feedback resistors for setting the output voltage. See the ADuM3471 data sheet and Figure 1 for details.

Layout considerations

The performance of this or any high-speed/high-resolution circuit is highly dependent on proper PCB layout, including but not limited to power supply bypassing, signal routing, and proper power and ground planes. For more information on PCB layout, see Tutorial MT-031 , Tutorial MT-101 , and A Practical Guide to High-Speed ​​Printed Circuit Board Layout (Analog Dialogue) .

For a complete design support package for the CN-0254, including schematics, board layout and BOM, see http://www.analog.com/CN0254-DesignSupport .

System performance

Figure 4 shows a plot of 10,000 ADC code samples (1 second at 1 kSPS) with CH0 to CH7 on the evaluation board terminal strip shorted to GND. Note that 95% of the code is at 4 LSB, with a peak-to-peak distribution of approximately 7 LSB. This corresponds to an rms value of approximately 7 ÷ 6.6 = 1.1 LSB.

The AC performance is shown in Figure 5. The sampling rate of 250 kSPS is controlled by the system demonstration platform ( EVAL-SDP-CB1Z SDP ), and the digital signal processing including signal windowing and FFT is calculated on the PC through the CN-0254 evaluation software. The input sine waveform is 20 kHz audio, provided by a low-distortion B&K sine generator Type 1051.