In most systems, there is a trade-off between performance and low power consumption. This circuit design focuses on examining some of the trade-offs while achieving low power (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 operational amplifier. This amplifier was chosen because it has excellent dynamic performance, operates from a single supply voltage, and provides rail-to-rail output. Additionally, 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.

The core of this circuit is the AD7988-1 16-bit, 100 kSPS successive approximation ADC, powered by a single VDD supply. It contains a low-power, high-speed, 16-bit sampling ADC and a multi-function 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 with 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 enables daisy-chaining of several ADCs onto a single three-wire bus. It is compatible with 1.8 V, 2.5 V, 3 V or 5 V logic when using the independent VIO supply pin.

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

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

The key feature that makes the ADA4841-1 ideal for single-supply applications is that it can be powered from a single supply rail in the application while connecting the negative supply rail to ground. The amplifier output can swing to within 50 mV of ground, which is acceptable 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 target signal range (0 V to 2.5 V), a 1 V headroom must be provided; therefore a 4 V supply rail is used in this circuit. The ADA4841-1 is available in a 6-pin SOT-23 or 8-pin SOIC package.

The 2.5 V reference voltage source used in this application is ADR4525 , which belongs to the adr45XX reference voltage source series. It can provide high accuracy, low power consumption, low noise, and has ±0.01% initial accuracy, excellent temperature stability and low output noise. . The ADR4525's low thermally induced output voltage hysteresis and low long-term output voltage drift improve system performance. The maximum operating current of 700 μA and the low dropout voltage of 500 mV (max) make this device ideal for portable equipment.

The three products used in this circuit are rated 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 the appropriate device selection among the many available products. The first step is to look at the different supply currents for three selected devices.

Typical supply currents calculated and measured for each contributing component are shown in Table 1. The VIO power supply for the ADC digital interface is negligible and therefore not included. Comparison of measured currents to calculated values ​​is very advantageous; passive components may cause small differences that make the supply current slightly different than typical data sheet specifications.

The AC performance of the AD7988-1 ADC is degraded when using low value reference voltages. This performance degradation is shown in Figure 2, where signal-to-noise ratio, signal-to-noise ratio (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 SNR performance is expected to be approximately 86 dB to 87 dB.

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