EVAL-AD7795EB datasheet(32/36 Pages) AD

32 / 36 page

AD7794/AD7795

Rev. D | Page 32 of 36

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 miscalled 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.

GAIN is the in-amp setting (1 to 128).

N = 24.

BURNOUT CURRENTS

The AD7794/AD7795 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 in the external

transducer circuit, and a measurement of the input voltage on

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

measured is full scale, the user needs to verify why this is the

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

is open circuit. It could also mean that the front-end sensor is

overloaded and is justified in outputting full scale, or that the

reference may be absent and the NOXREF bit is set, thus

clamping the data to all 1s.

When reading all 1s from the output, the user needs to check

these three cases before making a judgment. If the voltage

measured is 0 V, it may 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.

EXCITATION CURRENTS

The AD7794/AD7795 also contain two matched, software

configurable, constant current sources that can be programmed

to equal 10 μA, 210 μA, or 1 mA. Both source currents from

AVDD are directed to either the IOUT1 or IOUT2 pin of the

device. These current sources are controlled via bits in the IO

direct the current sources to IOUT1 or IOUT2, along with

selecting the value of the current. These current sources can be

used to excite external resistive bridge or RTD sensors.

BIAS VOLTAGE GENERATOR

A bias voltage generator is included on the AD7794/AD7795. It

biases the negative terminal of the selected input channel to

AVDD/2. This function is available on inputs AIN1(−) to

AIN3(−). It is useful in thermocouple applications, as the

voltage generated by the thermocouple must be biased about

some dc voltage if the gain is greater than 2. This is necessary

because the instrumentation amplifier requires headroom. If

there is no headroom, signals close to GND or AVDD do not

convert accurately.

The bias voltage generator is controlled using the VBIAS1 and

VBIAS0 bits in conjunction with the BOOST bit in the

configuration register. The power-up time of the bias voltage

generator is dependent on the load capacitance. To accommodate

higher load capacitances, each AD7794/AD7795 has a BOOST

bit. When this bit is set to 1, the current consumed by the bias

voltage generator is increased so that power-up time is reduced

considerably. Figure 11 shows the power-up times when

BOOST equals 0 and BOOST equals 1 for different load

capacitances. The current consumption of the AD7794/AD7795

increases by 40 μA when the bias voltage generator is enabled,

and BOOST equals 0. With the BOOST function enabled, the

current consumption increases by 250 μA.

REFERENCE

The AD7794/AD7795 have embedded 1.17 V references. These

references can be used to supply the ADC or external references

can be applied. The embedded references are low noise, low

drift references with 4 ppm/°C drift typically. For external

references, the ADC has a fully differential input capability for

the channel. In addition, the user has the option of selecting one

of two external reference options (REFIN1 or REFIN2). The

reference source for the AD7794/AD7795 is selected using the

REFSEL1 and REFSEL0 bits in the configuration register. When

the internal reference is selected, it is internally connected to

the modulator (it is not available on the REFIN pins).

The common-mode range for these differential inputs is from

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

excessive R-C source impedances introduce gain errors. The

reference voltage REFIN (REFIN(+) − REFIN(−)) is 2.5 V

nominal, but the AD7794/AD7795 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

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