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AD6672

Rev. 0 | Page 16 of 32

THEORY OF OPERATION

The AD6672 can sample any fS/2 frequency segment from dc to

250 MHz using appropriate low-pass or band-pass filtering at

the ADC inputs with little loss in ADC performance.

Programming and control of the AD6672 are accomplished

using a 3-pin, SPI-compatible serial interface.

ADC ARCHITECTURE

The AD6672 architecture consists of a front-end sample-and-

hold circuit, followed by a pipelined switched-capacitor ADC.

The quantized outputs from each stage are combined into a

final 11-bit result in the digital correction logic. The pipelined

architecture permits the first stage to operate on a new input

sample and the remaining stages to operate on the preceding

samples. Sampling occurs on the rising edge of the clock.

Each stage of the pipeline, excluding the last, consists of a low

resolution flash ADC connected to a switched-capacitor digital-

to-analog converter (DAC) and an interstage residue amplifier

(MDAC). The MDAC magnifies the difference between the

reconstructed DAC output and the flash input for the next stage

in the pipeline. One bit of redundancy is used in each stage to

facilitate digital correction of flash errors. The last stage simply

consists of a flash ADC.

The input stage of the AD6672 contains a differential sampling

circuit that can be ac- or dc-coupled in differential or single-

ended modes. The output staging block aligns the data, corrects

errors, and passes the data to the output buffers. The output

buffers are powered from a separate supply, allowing digital

output noise to be separated from the analog core. During

power-down, the output buffers go into a high impedance state.

The AD6672 features a noise shaping requantizer (NSR) to

allow higher than 11-bit SNR to be maintained in a subset of

the Nyquist band.

ANALOG INPUT CONSIDERATIONS

The analog input to the AD6672 is a differential switched-

capacitor circuit that has been designed to attain optimum

performance when processing a differential input signal.

The clock signal alternatively switches the input between

sample mode and hold mode (see the configuration shown in

Figure 24). When the input is switched into sample mode, the

signal source must be capable of charging the sampling

capacitors and settling within 1/2 clock cycle.

A small resistor in series with each input can help reduce the

peak transient current required from the output stage of the

driving source. A shunt capacitor can be placed across the

inputs to provide dynamic charging currents. This passive

network creates a low-pass filter at the ADC input; therefore,

the precise values are dependent on the application.

In intermediate frequency (IF) undersampling applications, the

shunt capacitors should be reduced. In combination with the

driving source impedance, the shunt capacitors limit the input

bandwidth. Refer to the AN-742 Application Note, Frequency

Domain Response of Switched-Capacitor ADCs; the AN-827

Application Note, A Resonant Approach to Interfacing Amplifiers

to Switched-Capacitor ADCs; and the Analog Dialogue article,

“Transformer-Coupled Front-End for Wideband A/D Converters,”

for more information on this subject.

CPAR1

CPAR2

CFB

BIAS

VIN+

VIN–

Figure 24. Switched-Capacitor Input

For best dynamic performance, match the source impedances

driving VIN+ and VIN− and differentially balance the inputs.

Input Common Mode

The analog inputs of the AD6672 are not internally dc biased.

In ac-coupled applications, the user must provide this bias

externally. Setting the device so that VCM = 0.5 × AVDD (or

0.9 V) is recommended for optimum performance. An on-

board common-mode voltage reference is included in the

design and is available from the VCM pin. Using the VCM

output to set the input common mode is recommended.

Optimum performance is achieved when the common-mode

voltage of the analog input is set by the VCM pin voltage

(typically 0.5 × AVDD). The VCM pin must be decoupled to

ground by a 0.1 μF capacitor, as described in the Applications

Information section. Place this decoupling capacitor close to the

pin to minimize the series resistance and inductance between

the part and this capacitor.

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