Achieving Low Power and High Performance in a 16-Bit, 100kSPS Data Acquisition System

Circuit Function and Advantages

In most systems, there is a trade-off between performance and low power consumption. The focus of this circuit design is to examine some of these trade-offs while achieving low power consumption (8 mW, typical) and high performance in a 16-bit, 100 kSPS data acquisition system.

This circuit uses the AD7988-1 low power (350 μA) PulSAR® analog-to-digital converter (ADC) driven directly from the ADA4841-1 high performance, low voltage, low power op amp. This amplifier was chosen because it has excellent dynamic performance, operates from a single supply voltage, and provides rail-to-rail outputs. In addition, the input common-mode voltage range includes the negative supply rail.

The AD7988-1 ADC requires an external reference voltage between 2.4 V and 5.1 V. In this application, the reference chosen is the ADR4525 precision 2.5 V reference.

Figure 1. Basic single-ended, low voltage, low power, 16-bit, 100 kSPS ADC solution.

Circuit Description

The heart of this circuit is the AD7988-1 16-bit, 100 kSPS successive approximation ADC, which operates from a single VDD supply. It contains a low power, high speed, 16-bit sampling ADC and a versatile serial port interface (SPI). On the rising edge of CNV, the device samples the analog input voltage difference between IN+ and IN-, ranging from 0 V to REF. The reference voltage (REF) is provided externally and can be independent of the supply voltage (VDD).

In the experiments performed for this circuit note, the AD7988-1 evaluation board was interfaced to the System Demonstration Platform (SDP, EVAL-SDP-CB1Z), and the ADC SPI-compatible serial interface was connected to the DSP SPORT interface. The ADC SPI interface is capable of daisy-chaining several ADCs onto a single 3-wire bus. It is compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using a separate VIO supply pin.

The AD7988-1 is available in a 10-lead MSOP or 10-lead QFN (LFCSP) package. For convenience, this board uses the MSOP package.

The ADC input is buffered and driven from the ADA4841-1 unity-gain stable, low noise and low distortion, rail-to-rail output amplifier, which typically operates at 1.1 mA of quiescent current. This amplifier offers low broadband voltage noise of 2.1 nV/√Hz and current noise of 1.4 pA/√Hz, with an excellent −105 dBc spurious-free dynamic range (SFDR) at 100 kHz. To maintain a low noise environment at lower frequencies, the amplifier has low 1/f noise of 7 nV/√Hz and 13 pA/√Hz at 10 Hz.

The main feature that makes the ADA4841-1 ideal for single-supply applications is that it can be powered from a single supply rail in this application while connecting the negative supply rail to ground. The amplifier output swings to within 50 mV of ground, which is an acceptable value for this application. Note that the input common-mode voltage range extends from the negative supply rail to within 1 V of the positive supply rail. To accommodate the signal range of interest (0 V to 2.5 V), 1 V of headroom must be provided; therefore, a 4 V supply rail is used in this circuit. The ADA4841-1 is available in a 6-lead SOT-23 or 8-lead SOIC package.

The 2.5 V reference used in this application is the ADR4525, which belongs to the ADR45xx voltage reference family and provides high precision, low power, low noise, and has ±0.01% initial accuracy, excellent temperature stability, and low output noise. The low thermally induced output voltage hysteresis and low long-term output voltage drift of the ADR4525 improve system performance. The maximum operating current of 700 μA and the low dropout voltage (maximum) of 500 mV make this device most suitable for portable equipment.

All three products used in this circuit are rated for operation over the full industrial temperature range of −40°C to +125°C.

Performance Expectations

Since power dissipation is critical in this application, it is necessary to analyze the contribution of each component to ensure that the appropriate device is selected among the many available. The first step is to look at the different supply currents of the three selected devices.

The calculated and measured typical supply currents for each contributing component are shown in Table 1. The VIO supply for the ADC digital interface is negligible and therefore not included. The measured currents compare very favorably to the calculated values; passive components can introduce small differences that can cause the supply currents to vary slightly from the typical data sheet specifications.

Table 1. Calculated and measured supply current contributions

The ac performance of the AD7988-1 ADC will degrade when using low value reference voltages. This degradation is shown in Figure 2, where the signal-to-noise ratio, signal-to-distortion and distortion (SINAD), and effective number of bits (ENOB) are shown as a function of the reference voltage. Note that for a 2.5 V reference voltage, the expected SNR performance is approximately 86 dB to 87 dB.

Figure 2. AD7988-1 ADC SNR, SINAD, and ENOB vs. reference voltage.

The measured results of the circuit are shown in Figure 3. The SNR performance of 86.17 dB is comparable to what would be expected from a 2.5 V reference voltage, as shown in Figure 2 above.

Figure 3. AC performance at 100 kSPS sampling rate measured with 10 kHz input tone, SNR = 86.17 dB

Common changes

Other pin-compatible 16-bit ADCs in the PulSAR® family offer higher sampling rates: AD7988-5 (500 kSPS), AD7980 (1 MSPS), and AD7983 (1.33 MSPS). Note that the higher the sampling rate, the higher the power consumption. Alternatively, if higher resolution is required, suitable pin-compatible devices are the AD7691 (18-bit, 250 kSPS), AD7690 (18-bit, 400 kSPS), AD7982 (18-bit, 1 MSPS differential input), AD7984 (18-bit, 1.33 MSPS).

For higher input voltage ranges, choose a higher reference voltage and higher voltage supply rails for the reference and ADC driver.

The dynamic performance of the AD7988-5 (16-bit, 500 kSPS) ADC under similar conditions, however, the sampling rate is 500 kSPS, is shown in Figure 4. The SNR is 86.37 dB.

AC performance at 500 kSPS sampling rate measured with 500 kSPS AD7988-5 ADC at 10 kHz input tone, SNR = 86.37 dB

Adding input common-mode voltage to bias the amplifier

In ac-coupled applications, the input signal must be biased so that it is centered within the ADC input range, which is 0 V to 2.5 V for a 2.5 V reference. The circuit shown in Figure 5 addresses this common-mode signal requirement.

Many amplifiers can be used for buffering purposes in this application. The AD8031 is a single-supply voltage feedback amplifier that offers high speed performance with a small signal bandwidth of 80 MHz, a slew rate of 30 V/µs, and a settling time of 125 ns. This amplifier is unity-gain stable with capacitive loads and consumes less than 2.5 mW on a single 3.3 V supply. The AD8031 is available in 5-lead SOT-23, 8-lead SOIC, 8-lead PDIP, and 8-lead MSOP packages. In this circuit, the AD8031 is used to buffer the 2.5 V reference voltage that arrives at the voltage divider that provides the required 1.25 V common-mode voltage to the input of the ADA4841-1. The additional power used by the buffer is shown in Table 2.

Figure 5. Enhanced circuit, including common-mode buffer, is used to center the input voltage range in an AC-coupled application.

Table 2. Calculated Supply Current Contributions Including VCM Buffer (AD8031)

Figure 6. Enhanced circuit including common-mode and reference buffers.

Adding a Reference Buffer

In applications where different circuits share a reference source, it may be necessary to buffer the reference voltage to ensure optimal performance. In this example, using the AD8032 (a dual-channel version of the AD8031) works very well, as shown in Figure 6. If the ADC reference inputs are buffered, the decoupling value can be reduced to a 10 μF ceramic chip capacitor placed as close to the device as possible.

Figure 7 and Figure 8 show the performance of the AD7988-1 and AD7988-5, respectively, while using the AD8032 amplifier to establish the VCM level and buffer the reference voltage, as shown in Figure 6. This circuit is implemented on the EVAL-CN0255-SDPZ evaluation board.

Figure 8. AC Performance Measured with 10 kHz Input Tone for Similar Configuration Using 500 kSPS， AD7988-5

CIRCUIT EVALUATION AND TEST

Equipment Needed （Equivalents Can Be Substituted）

EVAL-CN0255-SDPZ

System Demonstration Board （ EVAL-SDP-CB1Z）

Function generator/signal source， such as Audio Precision SYS-2522 used in these tests.

Power supply， 2.5 V and 4 V

PC with USB port， USB cable， and installed 10-lead PulSAR software

Setup and Test

The block diagram of the ac performance measurement setup is shown in Figure 9. Connect the 2.5 V and 4 V power supply to the evaluation board power terminal.

To measure the frequency response， connect the equipment as shown in Figure 9. Set the Audio Precision SYS-2522 signal generator for a 10 kHz frequency and a 2.5 V p-p sine wave with a 1.25 V dc offset. Record the data using the evaluation board software.

The software analysis is part of the evaluation board software that allows the user to capture and analyze ac and dc performance.

Figure 9. Circuit Test Setup for Measuring AC Performance