High Speed, Precision, Differential AC-Coupled Drive Circuit for 16-Bit, 6 MSPS AD7625 PulSAR ADC
Figure 1 shows that the differential ac-coupled source has signals 180° out of phase with respect to each other, and the voltage swings around ground on each input. In the test setup, an Audio Precision AP2700-series generator was used to generate the differential input signals. Two 10 μF NP0 capacitors are used to couple the signal into the driver circuit. The two ADA4899-1 amplifiers are connected in a unity gain noninverting configuration (ADA4899-1 is unity gain stable) to condition the analog input to the AD7625 inputs, thereby providing sufficient isolation from the converter switched capacitor transients and also setting the correct common-mode input voltage. For the AD7625, the common-mode voltage is one-half the internal reference voltage, REF/2, where REF = 4.096 V.
Figure 1: AC-Coupled Differential Drive Circuit for the AD7625 ADC (Simplified Schematic: All Connections and Decoupling Not Shown)
The VCM output pin of the AD7625 is a useful function that provides one-half the reference voltage used internally by the AD7625. This VCM output is buffered with the AD8031 rail-to-rail amplifier, thereby providing a precise common-mode voltage for the analog input amplifiers.
Note that the feedback pin (FB) on the ADA4899-1 is internally connected to its output pin, thereby minimizing parasitic capacitance and inductance. The ADA4899-1 also has an exposed pad for heat dissipation which should be electrically connected to the ground plane.
In order to allow sufficient headroom at the output of the ADA4899-1 op amps when they go to +4.096 V and 0 V, both devices are powered with a +7 V and −5 V supply. Because the amplifier and the ADC operate on different supply voltages, protection circuits may be required at the ADC inputs as described in Tutorial MT-036.
Excellent layout, grounding, and decoupling techniques must be utilized in order to achieve the desired performance from the circuits discussed in this note (see Tutorial MT-031 and Tutorial MT-101). As a minimum, a 4-layer PCB should be used with one ground plane layer, one power plane layer, and two signal layers. The AD7625 data sheet also includes a section on layout and decoupling practices for the device.
Figure 2 and Figure 3 show the excellent distortion and noise performance obtained with the circuit.
Figure 2: FFT Output with a 2 kHz Input Tone and a Sampling Rate of 6 MSPS
Figure 3: Expanded View of the FFT with a 2 kHz Input Tone and a Sampling Rate of 6 MSPS
|AD8031||2.7 V, 800 µA, 80 MHz Rail-to-Rail I/O Single Amplifier||
|ADP3334||High Accuracy Low IQ, 500 mA anyCAP® Adjustable Low Dropout Regulator||
|AD7625||16-Bit, 6MSPS PulSAR Differential ADC||
|ADR434||Ultralow Noise XFET® 4.096V Voltage Reference w/Current Sink and Source Capability||
|ADA4899-1||Unity-Gain Stable, Ultralow Distortion, 1 nV/√Hz Voltage Noise, High Speed Op Amp||