CN0080

High Speed, Precision, Differential AC-Coupled Drive Circuit for 16-Bit, 6 MSPS AD7625 PulSAR ADC
Manufactured by:

Circuit Function & Benefits

X+
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

X+
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.


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

Sample Products

X+

Samples

Product

Description

Available Product
Models to Sample

AD8031 2.7 V, 800 µA, 80 MHz Rail-to-Rail I/O Single Amplifier

AD8031ANZ

AD8031ARTZ-REEL7

AD8031ARZ

AD8031BNZ

AD8031BRZ

ADP3334 High Accuracy Low IQ, 500 mA anyCAP­® Adjustable Low Dropout Regulator

ADP3334ACPZ-REEL7

ADP3334ARMZ-REEL7

ADP3334ARZ

AD7625 16-Bit, 6MSPS PulSAR Differential ADC

AD7625BCPZ

ADR434 Ultralow Noise XFET® 4.096V Voltage Reference w/Current Sink and Source Capability

ADR434ARMZ

ADR434ARZ

ADA4899-1 Unity-Gain Stable, Ultralow Distortion, 1 nV/√Hz Voltage Noise, High Speed Op Amp

ADA4899-1YCPZ-R7

ADA4899-1YRDZ