CIRCUIT OVERVIEW

CIRCUIT FUNCTION AND BENEFITS

This circuit provides a method to drive an ac-coupled differential input signal to the AD7625, 16-bit, 6 MSPS PulSAR® differential ADC. This circuit has been designed to ensure maximum performance of the AD7625 by providing adequate settling time and low distortion. It uses a buffered VCM output voltage from the AD7625 to set each amplifier's common-mode level.

CIRCUIT DESCRIPTION

The signal source applied to the AD7625 should be buffered to enable driving the AD7625 switch capacitor front end and maintain low distortion. The ADA4899-1 used on each input provides the required drive, distortion, and settling time to maximize the performance of the AD7625 16-bit, 6 MSPS 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.

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.

SAMPLE PRODUCTS USED IN THIS CIRCUIT