The circuit in Figure 1 uses a 16-bit, 6 MSPS successive approximation (SAR) analog-to-digital converter (ADC) and differential-to-differential driver combination to target low noise at low power (signal-to-noise ratio [SNR] = 88.6 dB) and low distortion (total harmonic distortion [THD] = −110 dBc). This circuit is ideally suited for high-performance multiplexed data acquisition systems, such as portable digital X-ray systems and security scanners, because the SAR architecture performs sampling without the delays or pipeline delays typically associated with pipelined ADCs. A 6 MSPS sampling rate enables fast sampling of multiple channels, and the ADC features true 16-bit DC linearity performance and a serial low-voltage differential signaling (LVDS) interface for low pin count and low digital noise.

The driver uses two low-noise (1 nV/√Hz) ADA4897-1 op amps to maintain the dynamic performance of the AD7625 ADC at low power levels (3 mA per amplifier) . The ADA4897-1 has a fast 0.1% settling time of 45ns, making it ideal for multiplexing applications.

This combination delivers industry-leading dynamic performance at low power in a small amount of board space, with the AD7625 available in a 5 mm × 5 mm, 32-lead LFCSP package; the ADA4897-1 available in an 8-lead SOIC package; and the AD8031 available in 5-pin SOT-23 package.

The ADA4897-1 features low distortion (−93 dB spurious-free dynamic range [SFDR] at 1 MHz), fast 0.1% settling time (36 ns), and high bandwidth (230 MHz, −3 dB, G = 1) . Both ADA4897-1 drivers are configured with a gain of 1. Single-pole 2.95 MHz low-pass RC filters using 20Ω resistors and 2.7 nF capacitors are placed between each driver and the ADC. This filter limits the op amp output noise at the input of the AD7625 and provides some attenuation of out-of-band harmonics.

Set the common-mode voltage at the output of the ADA4897-1 by buffering the AD7625's V _CM output voltage (nominally 2.048 V) using the AD8031 configured as a unity-gain buffer . The common-mode bias voltage is applied to the input through a 590Ω series resistor. The AD8031 is ideal for driving common-mode voltages because of its low output impedance and its fast settling when transient currents occur.

The AD7625 uses an LVDS interface to achieve industry-breaking dynamic performance, with a signal-to-noise ratio of 92 dB (6 MSPS) and 16-bit (1 LSB) integral nonlinearity (INL) performance. The ADR434 voltage reference (4.096 V) is a low-noise, high-precision XFET voltage reference with low temperature drift. Its source current output is up to 30 mA, and its maximum current sink capability is 20 mA.

The ADR434 is available in an 8-lead MSOP or 8-lead narrow body SOICC package. The AD8031 op amp isolates the ADR434 output from the AD7625 reference input, providing low impedance and fast settling for transient currents at the REF input.

The dual drivers require only 54 mW, and combined with the 135 mW of ADC power, 12 mW of reference and buffer, the entire circuit results in a total power consumption of only 201 mW.

The circuit uses +7 V and −2 V supplies for the inputs of the ADA4897-1 driver to minimize power consumption and achieve optimal system distortion performance. The ADA4897-1 output stage is rail-to-rail and swings between 150 mV and 4.85 V from a single 5 V supply. However, an additional 2 V of headroom at both ends of the range provides low distortion.

Figure 2 shows the ac performance of the circuit using +7 V and −2 V supplies for the input stage. SNR = 88.6 dB, THD = −110.7 dB, 20 kHz input signal is 0.6 dB below full scale (93% full scale).

Figure 3 shows the AC performance of the circuit using a single 5 V supply for the input stage. SNR = 86.7dB, THD = -101.1 dB, 20 kHz input signal is 1.55 dB below full scale (84% full scale).

When the supply voltage is reduced from −2 V, +7 V to 0 V, +5V, the data shows that the SNR is reduced by approximately 1.9 dB and the THD is reduced by approximately 9.6 dB.

The single power supply configuration is suitable for users whose system does not have dual power supplies but still needs high performance.