7

®

ADC700

measurements, code transition values are the locations actu-

ally measured for this spec. The ideal gain is V

Gain Error is expressed in % (of reading). See Figure 3.

Gain Error of the ADC700 may be trimmed to zero using

external trim potentiometers.

Offset Error

Unipolar Offset Error—The deviation of the actual code-

midpoint value of the first code from the ideal value located

at 1/2LSB below the ideal first transition value (i.e. at zero

volts).

Bipolar Offset Error—The deviation of the actual code-

midpoint of the first code from the ideal value located

at 1/2LSB below the ideal first transition value located at –

Again, transition values are the actual measured parameters.

Offset and Zero errors of the ADC700 may be trimmed to

zero using external trim potentiometers. Offset Error is

expressed as a percentage of FSR.

Bipolar Zero Error—The deviation of the actual mid-

scale-code midpoint value from zero. Transition values are

the actual measured parameter and it is 1/2 LSB below zero

volts. The error is comprised of Bipolar Offset Error, 1/2 the

Gain Error, and the Linearity Error of bit 1. Bipolar Zero

Error is expressed as a percentage of FSR.

Power Supply Sensitivity

Power Supply Sensitivity describes the maximum change in

the full-scale transition value from the initial value for a

change in each power supply voltage. PSR is specified in

units of %FSR/% change in each supply voltage.

The major effect of power supply voltage deviations from

the rated values will be a small change in the Gain (scale

factor). Power Supply Sensitivity is also a function of ripple

frequency. Figure 4 illustrates typical Power Supply Sensi-

tivity performance of ADC700 versus ripple frequency.

INSTALLATION

POWER SUPPLY SELECTION

Linear power supplies are preferred. Switching power sup-

ply specifications may appear to indicate low noise output,

but these specifications are rms specs. The spikes generated

in switchers may be hard to filter. Their high-frequency

components may be extremely difficult to keep out of the

power supply return system. If switchers must be used, their

outputs must be carefully filtered and the power supply itself

should be shielded and located as far away as possible from

precision analog circuits.

LAYOUT CONSIDERATIONS

Because of the high resolution and linearity of the ADC700,

system design problems such as ground path resistance and

contact resistance become very important. For a 16-bit

resolution converter with a +10V Full-Scale Range, 1LSB is

153

µV. Circuit situations that cause only second- or third-

order errors in 8-, 10-, or 12-bit A/D converters can induce

first-order errors in 16-bit resolution devices.

Power Supply Wiring

Use heavy power supply and power supply common (ground)

wiring. A ground plane is usually the best solution for

preserving dynamic performance and reducing noise cou-

pling into sensitive converter circuits.

When passing converter power through a connector, use

every available spare pin for making power supply return

connections, and use some of the pins as a Faraday shield to

separate the analog and digital common lines.

Power Supply Returns

(Analog Common and Digital Common)

Connect Analog Common and Digital Common together

right at the converter with the ground plane. This will usually

give the best performance. However, it may cause problems

for the system designer. Where it is absolutely necessary to

separate analog and digital power supply returns, each should

be separately returned to the power supply. Do not connect

Analog Common and Digital Common together and then run

a single wire to the power supply. Connect a 1 to 47

µF

tantalum capacitor between Digital Common and Analog

Common pins as close to the package as possible.

Power Supply Bypassing

Every power-supply line leading into an A/D converter must

be bypassed to its common pin. The bypass capacitor should

be located as close to the converter package as possible and

tied to a solid ground—connecting the capacitors to a noisy

ground defeats the purpose of the bypass. Use tantalum

capacitors with values of from 10

µF to 100µF and parallel

them with smaller ceramic capacitors for high frequency

filtering if necessary.

Separate Analog and Digital Signals

Digital signals entering or leaving the layout should have

minimum length to minimize crosstalk to analog wiring.

Keep analog signals as far away as possible from digital

signals. If they must cross, cross them at right angles.

Coaxial cable may be necessary for analog inputs in some

situations.

Shield Other Sensitive Points

The most critical of these is the comparator input (pin 1). If

this pin is not used for offset adjustment, then it should be

surrounded with ground plane or low-impedance power

FIGURE 4. Power Supply Rejection Ripple vs Frequency.

1

Frequency (Hz)

10

100

1k

10k

100k

0.1

0.01

0.001

–V

CC

+V

DD

+V

CC