AD7190 데이터 시트보기 (PDF) - Analog Devices

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AD7190

4.8 kHz Ultralow Noise 24-Bit Sigma-Delta ADC with PGA

Analog Devices 

AD7190

The internal clock can also be made available at the MCLK2

pin. This is useful when several ADCs are used in an application

and the devices need to be synchronized. The internal clock

from one device can be used as the clock source for all ADCs

in the system. Using a common clock, the devices can be syn-

chronized by applying a common reset to all devices, or the

SYNC pin can be pulsed.

BURNOUT CURRENTS

The AD7190 contains two 500 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 (burn) bit in the

configuration register. These currents can be used to verify that

an external transducer remains operational before attempting to

take measurements on that channel. After 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 resultant 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 the reference may be absent

and the NOREF bit in the status register 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. The current sources work over the normal absolute

input voltage range specifications when the analog inputs are

buffered and chop is disabled.

REFERENCE

The ADC has a fully differential input capability for the refer-

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

one of two external reference options (REFIN1(x) or REFIN2(x)).

The reference source for the AD7190 is selected using the

REFSEL bit in the configuration register. The REFIN2(x) pins

are dual purpose: they can function as two general-purpose

output pins or as reference pins. When the REFSEL bit is set

to 1, these pins automatically function as reference 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 (REFINx(+) − REFINx(−)) is AVDD

nominal, but the AD7190 is functional with reference voltages

from 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 AD7190 is used in a nonratiometric applica-

tion, use a low noise reference.

Recommended 2.5 V reference voltage sources for the AD7190

include the ADR421 and ADR431, which are low noise

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 combinations on

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

impedance of the source driving the reference inputs.

Reference voltage sources like those recommended above (for

example, ADR431) typically have low output impedances and

are, therefore, tolerant to having decoupling capacitors on

REFINx(+) 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 REFINx pins is not

recommended in this type of circuit configuration.

REFERENCE DETECT

The AD7190 includes on-chip circuitry to detect whether the

part has a valid reference for conversions or calibrations. This

feature is enabled when the REFDET bit in the configuration

register is set to 1. If the voltage between the selected REFINx(+)

and REFINx(–) pins goes below 0.3 V or either the REFINx(+)

or REFINx(–) input is open circuit, the AD7190 detects that it

no longer has a valid reference. In this case, the NOREF bit of

the status register is set to 1. If the AD7190 is performing normal

conversions and the NOREF bit becomes active, the conversion

results is 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 con-

version result read from the ADC data register is all 1s. If the

AD7190 is performing either an offset or 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 AD7190 can be reset by

writing consecutive 1s to the device; 40 consecutive 1s are

required to perform the reset. This resets the logic, the digital

filter, and the analog modulator, whereas all on-chip registers

are reset to their default values. A reset is automatically

performed on power-up. When a reset is initiated, the user

must allow a period of 500 μs before accessing any of the on-

chip registers. A reset is useful if the serial interface becomes

asynchronous due to noise on the SCLK line.

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