AD10242

–12–

USING THE FLEXIBLE INPUT

The AD10242 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. While the standard inputs are

±0.5 V, ±1.0 V, and

±2.0 V, the user can select the input impedance of the AD10242

on any input by using the other inputs as alternate locations for

GND or an external resistor. The following chart summarizes the

impedance options available at each input location:

Ω when A

Ω when A

Ω when A

Ω when A

Ω, with A

Ω when A

=

100

Ω.

Ω when A

Ω to GND.

Ω when A

Ω to GND.

While the analog inputs of the AD10242 are designed for

dc- coupled bipolar inputs, the AD10242 has the ability to

use unipolar inputs in a user selectable mode through the addi-

tion of an external resistor. This allows for 1 V, 2 V, and 4 V

full-scale unipolar signals to be applied to the various inputs

Ω resis-

tor (typical, offset calibration required) between UPOS and

UCOM shifts the reference voltage setpoint to allow a unipolar

positive voltage to be applied at the inputs of the device. To cali-

brate offset, apply a midscale dc voltage to the converter while

adjusting the unipolar resistor for a midscale output transition.

UPOS

AD10242

2.43k

UCOM

Figure 13. Unipolar Positive

To operate with –1 V, –2 V, or –4 V full-scale unipolar signals,

place a 2.67 k

Ω resistor (typical, offset calibration required)

between UNEG and UCOM. This again shifts the reference volt-

age setpoint to allow a unipolar negative voltage to be applied at

the inputs of the device. To calibrate offset, apply a midscale dc

voltage to the converter while adjusting the unipolar resistor for

a midscale output transition.

UNEG

AD10242

2.67k

UCOM

Figure 14. Unipolar Negative

GROUNDING AND DECOUPLING

Analog and Digital Grounding

Proper grounding is essential in any high speed, high resolution

system. Multilayer printed circuit boards (PCBs) are recom-

mended to provide optimal grounding and power schemes. The

use of ground and power planes offers distinct advantages:

1. The minimization of the loop area encompassed by a signal

and its return path.

2. The minimization of the impedance associated with ground

and power paths.

3. The inherent distributed capacitor formed by the power

plane, PCB insulation, and ground plane.

These characteristics result in both a reduction of electro-

magnetic interference (EMI) and an overall improvement in

performance.

It is important to design a layout that prevents noise from cou-

pling to the input signal. Digital signals should not be run in

parallel with input signal traces and should be routed away from

the input circuitry. The AD10242 does not distinguish between

analog and digital ground pins as the AD10242 should always

be treated like an analog component. All ground pins should be

connected together directly under the AD10242. The PCB

should have a ground plane covering all unused portions of the

component side of the board to provide a low impedance path

and manage the power and ground currents. The ground plane

should be removed from the area near the input pins to reduce

stray capacitance.

LAYOUT INFORMATION

The schematic of the evaluation board (Figure 15) represents a

typical implementation of the AD10242. The pinout of the

AD10242 is very straightforward and facilitates ease of use

and the implementation of high frequency/high resolution

design practices. It is recommended that high quality ceramic

chip capacitors be used to decouple each supply pin to ground

directly at the device. All capacitors except the one placed on

ENCODE can be standard high quality ceramic chip capacitors.

The capacitor used on the

ENCODE pin must be a low induc-

tance chip capacitor as referenced previously.

REV. D