AD13280SLASHPCB datasheet(14/28 Pages) AD

14 / 28 page

AD13280

Rev. C | Page 14 of 28

THEORY OF OPERATION

The AD13280 is a high dynamic range 12-bit, 80 MHz pipeline

delay (three pipelines) analog-to-digital converter (ADC). The

custom analog input section provides input ranges of 1 V p-p

and 2 V p-p and input impedance configurations of 50 Ω, 100 Ω,

and 200 Ω.

The AD13280 employs four monolithic Analog Devices com-

ponents per channel (AD8045, AD8138, AD8031, and a custom

ADC IC), along with multiple passive resistor networks and

decoupling capacitors to fully integrate a complete 12-bit

analog-to-digital converter (ADC).

In the single-ended input configuration, the input signal is passed

through a precision laser-trimmed resistor divider, allowing the

user to externally select operation with a full-scale signal of ±0.5 V

or ±1.0 V by choosing the proper input terminal for the applica-

tion. The result of the resistor divider is to apply a full-scale

input of approximately 0.4 V to the noninverting input of the

internal AD8045 amplifier.

The AD13280 analog input includes an AD8045 amplifier

featuring an innovative architecture that maximizes the dynamic

range capability on the amplifier inputs and outputs. The AD8045

amplifier provides a high input impedance and gain for driving the

AD8138 in a single-ended to differential amplifier configuration.

The AD8138 has a −3 dB bandwidth at 300 MHz and delivers a

differential signal with the lowest harmonic distortion available in

a differential amplifier. The AD8138 differential outputs help

balance the differential inputs to the custom ADC, maximizing

the performance of the device.

The AD8031 provides the buffer for the internal reference

analog-to-digital converter. The internal reference voltage of

the custom ADC is designed to track the offsets and drifts and

is used to ensure matching over an extended temperature range

of operation. The reference voltage is connected to the output

common-mode input on the AD8138. This reference voltage

sets the output common mode on the AD8138 at 2.4 V, which

is the midsupply level for the ADC.

The custom ADC has complementary analog input pins, AIN

and AIN. Each analog input is centered at 2.4 V and should

swing ±0.55 V around this reference. Because AIN and AIN are

180 degrees out of phase, the differential analog input signal is

2.2 V peak-to-peak. Both analog inputs are buffered prior to

the first track-and-hold.

The custom ADC digital outputs drive 100 Ω series resistors

(see Figure 16). The result is a 12-bit, parallel digital CMOS-

compatible word, coded as a twos complement.

USING THE SINGLE-ENDED INPUT

The AD13280 has been designed with user ease of operation in

mind. Multiple input configurations have been included on-

board to allow the user a choice of input signal levels and input

impedance. The standard inputs are ±0.5 V and ±1.0 V. The

user can select the input impedance of the AD13280 on any

input by using the other inputs as alternate locations for the

GND. The following is a summary of the impedance options

available at each input location:

AMP-IN-x-1 = 100 Ω when AMP-IN-x-2 is open.

AMP-IN-x-1 = 50 Ω when AMP-IN-x-2 is shorted to GND.

AMP-IN-x-2 = 200 Ω when AMP-IN-x-1 is open.

Each channel has two analog inputs: AMP-IN-A-1 and

AMP-IN-A-2 or AMP-IN-B-1 and AMP-IN-B-2. Use

AMP-IN-A-1 or AMP-IN-B-1 when an input of ±0.5 V full

scale is desired. Use AMP-IN-A-2 or AMP-IN-B-2 when ±1 V

full scale is desired. Each channel has an AMP-OUT that must

be tied to either a noninverting or inverting input of a

differential amplifier with the remaining input grounded. For

example, Side A, AMP-OUT-A (Pin 6) must be tied to A+IN

(Pin 5) with A−IN (Pin 4) tied to ground for noninverting

operation or AMP-OUT-A (Pin 6) tied to A−IN (Pin 4) with

A+IN (Pin 5) tied to ground for inverting operation.

USING THE DIFFERENTIAL INPUT

Each channel of the AD13280 is designed with two optional

differential inputs, A+IN, A−IN and B+IN, B−IN. The inputs

provide system designers with the ability to bypass the AD8045

amplifier and drive the AD8138 directly. The AD8138 differen-

tial ADC driver can be deployed in either a single-ended or

differential input configuration. The differential analog inputs

have a nominal input impedance of 620 Ω and nominal full-

scale input range of 1.2 V p-p. The AD8138 amplifier drives a

differential filter and the custom analog-to-digital converter.

The differential input configuration provides the lowest even-

order harmonics and signal-to-noise (SNR) performance

improvement of up to 3 dB (SNR = 73 dBFS). Exceptional care

was taken in the layout of the differential input signal paths.

The differential input transmission line characteristics are

matched and balanced. Equal attention to system level signal

paths must be provided in order to realize significant perform-

ance improvements.

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