EVAL-AD7799EBZ datasheet(23/28 Pages) AD

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Data Sheet

AD7798/AD7799

Rev. B | Page 23 of 28

DATA OUTPUT CODING

When the ADC is configured for unipolar operation, the output

code is natural (straight) binary with a zero differential input

voltage resulting in a code of 00...00, a midscale voltage resulting

in a code of 100...000, and a full-scale input voltage resulting in

a code of 111...111. The output code for any analog input voltage

can be represented as

Code = (2N × AIN × GAIN)/VREF

When the ADC is configured for bipolar operation, the output

code is offset binary, with a negative full-scale voltage resulting

in a code of 000...000, a zero differential input voltage resulting

in a code of 100...000, and a positive full-scale input voltage

resulting in a code of 111...111. The output code for any analog

input voltage can be represented as

Code = 2N–1 × [(AIN × GAIN/VREF) + 1]

where:

AIN is the analog input voltage.

N = 16 for the AD7798, and N = 24 for the AD7799.

BURNOUT CURRENTS

The AD7798/AD7799 each contain two 100 nA constant

current generators—one sourcing current from AVDD to

AIN(+), and one sinking current from AIN(−) to GND. The

currents are switched to the selected analog input pair. Both

currents are either on or off, depending on the burnout current

enable (BO) bit in the configuration register. These currents can

be used to verify that an external transducer is still operational

before attempting to take measurements on that channel. Once

the burnout currents are turned on, they flow into the external

transducer circuit, and a measurement of the input voltage on

the analog input channel can be taken. If the resultant voltage

measured is full scale, the user must determine why this is the

case. A full-scale reading could mean that the front-end sensor

is open circuit, that the front-end sensor is overloaded and is

justified in outputting full scale, or that the reference is absent

and, thus, clamping the data to all 1s.

When reading all 1s from the output, the user should check

these three cases before making a judgment. If the voltage

measured is 0 V, it might indicate that the transducer has short-

circuited. For normal operation, these burnout currents are

turned off by writing a 0 to the BO bit in the configuration

input voltage range specifications with buffers on.

REFERENCE

The common-mode range for these differential inputs is from

GND to AVDD. The reference input is unbuffered; therefore,

excessive resistance/capacitance source impedances introduce

gain errors. The reference voltage REFIN (REFIN(+) − REFIN(−))

is 2.5 V nominal, but the AD7798/AD7799 are functional with

reference voltages from 0.1 V to AVDD. In applications where the

excitation (voltage or current) for the transducer on the analog

input also drives the reference voltage for the part, the effect of

the low frequency noise in the excitation source is removed

because the application is ratiometric. If the AD7798/AD7799

are used in a nonratiometric application, a low noise reference

should be used.

Recommended 2.5 V reference voltage sources for the AD7798/

AD7799 include the ADR381 and ADR391, which are low noise,

low power references. Also note that the reference inputs provide

a high impedance, dynamic load. Because the input impedance

of each reference input is dynamic, resistor/capacitor combina-

tions on these inputs can cause dc gain errors, depending on the

output impedance of the source driving the reference inputs.

Reference voltage sources such as those recommended above

(for example, ADR391) typically have low output impedances

and are, therefore, tolerant to having decoupling capacitors on

REFIN(+) without introducing gain errors in the system.

Deriving the reference input voltage across an external resistor

means that the reference input sees a significant external source

impedance. External decoupling on the REFIN pins is not

recommended in this type of circuit configuration.

REFERENCE DETECT

The AD7798/AD7799 include on-chip circuitry to detect if

there is a valid reference for conversions or calibrations. This

feature is enabled when the REF_DET bit in the configuration

REFIN(–) pins goes below 0.3 V, or either the REFIN(+) or

REFIN(–) inputs are open circuit, the AD7798/AD7799 detect

that there is no longer a valid reference. In this case, the NOREF

bit of the status register is set to 1. If the AD7798/AD7799 are

performing normal conversions and the NOREF bit becomes

active, the conversion results revert to all 1s. Therefore, it is not

necessary to continuously monitor the status of the NOREF bit

when performing conversions. It is only necessary to verify its

status if the conversion result read from the ADC data register

is all 1s. If the AD7798/AD7799 are performing an offset of full-

scale calibration and the NOREF bit becomes active, the updating

of the respective calibration registers is inhibited to avoid loading

incorrect coefficients to these registers, and the ERR bit in the

status register is set. If the user is concerned about verifying that

a valid reference is in place every time a calibration is performed,

the status of the ERR bit should be checked at the end of the

calibration cycle.

RESET

The circuitry and serial interface of the AD7798/AD7799 can

be reset by writing 32 consecutive 1s to the device. This resets

the logic, the digital filter, and the analog modulator, and all

on-chip registers are reset to their default values. A reset is

automatically performed upon power-up. When a reset is

initiated, the user must allow a period of 500 µs before

accessing an on-chip register. A reset is useful if the serial

interface becomes asynchronous due to noise on the SCLK line.

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