REV. 0

AD13280

–9–

AMP-IN-X-1

100

100

TO AD8037

AMP-IN-X-2

Figure 2. Single-Ended Input Stage

LOADS

LOADS

ENCODE

10k

10k

ENCODE

10k

10k

Figure 3. ENCODE Inputs

CURRENT MIRROR

CURRENT MIRROR

DR OUT

Figure 4. Digital Output Stage

CURRENT MIRROR

CURRENT MIRROR

D0–D11

100

Figure 5. Digital Output Stage

THEORY OF OPERATION

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

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

analog input section provides input ranges of 1 V and 2 V p-p

and input impedance configurations of 50

Ω, 100 Ω, and 200 Ω.

The AD13280 employs four monolithic ADI components per

channel (AD8037, 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.

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

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

scale input approximately 0.4 V to the noninverting input of the

internal AD8037 amplifier.

The AD13280 analog input includes an AD8037 amplifier featur-

ing an innovative architecture that maximizes the dynamic range

capability on the amplifiers’ inputs and outputs. The AD8037

amplifier provides a high input impedance and gain for driving

the AD8138 in a single-ended to differential amplifier configura-

tion. 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. Since 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 (Fig-

ure 5). The result is a 12-bit parallel digital CMOS-compatible

word, coded as two’s complement.

USING THE SINGLE-ENDED INPUT

The AD13280 has been designed with the user’s ease of opera-

tion 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 chart summarizes 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