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參數資料
| 型號： | AD7719BRU |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | Low Voltage, Low Power, Factory-Calibrated 16-/24-Bit Dual ADC |
| 中文描述： | 6-CH 24-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDSO28 |
| 封裝： | MS-153AE, TSSOP-28 |
| 文件頁數： | 33/40頁 |
| 文件大?。?/td> | 367K |
| 代理商： | AD7719BRU |
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REV. 0
AD7719
–33–
Data Output Coding
When the ADC is configured for unipolar operation, the output
coding is natural (straight) binary with a zero differential input
voltage resulting in a code of 000 . . . 000, 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 on the main ADC can be represented as follows:
Code
= (
AIN
×
GAIN
×
2
N
)/(1.024
×
V
REF
)
Where
AIN
is the analog input voltage,
GAIN
is the PGA gain,
i.e., 1 on the 2.56 V range and 128 on the 20 mV range, and
N = 16 in 16-bit mode and N = 24 in 24-bit mode of operation.
The output code for any analog input voltage on the aux ADC
can be represented as follows:
Code =
(
AIN
×
GAIN
×
2
N
)
/V
REF
Where
AIN
is the analog input voltage,
GAIN
is one or two deter-
mined by the ARN bit in the aux ADC control register, i.e., 1 on
the VREF range and 2 on the VREF/2 range and N = 16.
When an ADC is configured for bipolar operation, the coding is
offset binary with a negative full-scale voltage resulting in a code
of 000 . . . 000, a zero differential voltage resulting in a code of
100 . . . 000, and a positive full-scale voltage resulting in a code
of 111 . . . 111. The output code from the main ADC for any
analog input voltage can be represented as follows:
Code = 2
N–1
×
[(
AIN
×
GAIN
/(1.024
×
V
REF
)) + 1]
Where
AIN
is the analog input voltage,
GAIN
in the PGA gain,
i.e., 1 on the
±
2.56 V range and 128 on the
±
20 mV range and
N = 16 in 16 bit mode and N = 24 in 24-bit mode of operation.
The output code from the aux ADC for any analog input volt-
age can be represented as follows:
Code
= 2
N
–
1
×
[(
AIN
×
GAIN/V
REF
) + 1]
Where
AIN
is the analog input voltage,
GAIN
is one or two deter-
mined by the ARN bit in the aux ADC control register, i.e., 1 on
the
±
VREF range, 2 on the
±
VREF/2 range, and N = 16.
Burnout Currents
The main ADC on the AD7719 contains two 100 nA constant
current generators, one sourcing current from AV
DD
to AIN(+),
and one sinking current from AIN(
–
) to AGND. 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 IOCON 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 cur-
rents are turned on, they will 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 NOXREF bit is set, thus clamping the data to all ones.
When reading all ones from the output the user needs to check
these three cases before making a judgement. 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 IOCON register.
The current sources work over the normal absolute input volt-
age range specifications with buffers on.
Excitation Currents
The AD7719 also contains two matched, software configurable
200
μ
A constant current-sources. Both source current from
AV
DD
that is directed to either the IOUT1 or IOUT2 pins of
the device. These current sources are controlled via bits in the
IOCON register. The configuration bits enable the current
sources and they can be configured to source 200
μ
A individu-
ally to both pins or a combination of both currents, i.e., 400
μ
A
to either of the selected output pins. These current sources can
be used to excite external resistive bridge or RTD sensors.
Crystal Oscillator
The AD7719 is intended for use with a 32.768 kHz watch crys-
tal. A PLL internally locks onto a multiple of this frequency to
provide a stable 4.194304 MHz clock for the ADC. The modu-
lator sample rate is the same as the crystal oscillator frequency.
The start-up time associated with 32 kHz crystals is typically
300 ms. The OSPD bit in the mode register can be used to
prevent the oscillator from powering down when the AD7719 is
placed in power-down mode. This avoids having to wait 300 ms
after exiting power-down to start a conversion at the expense of
raising the power-down current.
Reference Input
The AD7719 has a fully-differential reference input capability
for the main channel while the auxiliary channel accepts only a
single-ended reference. On the main channel, the reference inputs
REFIN1(+) and REFIN1(
–
) provide a differential reference
input capability. The common-mode range for these differential
inputs is from AGND to AV
DD
. The reference input is unbuffered
and therefore excessive R-C source impedances will introduce
gain errors. The nominal reference voltage, VREF, (REFIN1(+)
–
REFIN1(
–
), for specified operation is 2.5 V, but the AD7719
is functional with reference voltages from 1 V to AV
DD
. In appli-
cations 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
will be removed as the application is ratiometric. If the AD7719
is used in a nonratiometric application, a low noise reference
should be used. Recommended reference voltage sources for
the AD7719 include the AD780, REF43, and REF192. It should
also be noted 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 that is driving the reference inputs. Reference
voltage sources like those recommended above (e.g., AD780), will
typically have low output impedances and are therefore tolerant
to having decoupling capacitors on the REFIN1(+) without
introducing gain errors in the system. Deriving the reference
input voltage across an external resistor, as shown in Figure 19,
will mean that the reference input sees a significant external
source impedance. External decoupling on the REFIN1(+)
and REFIN1(
–
) pins would not be recommended in this type
of circuit configuration.
The auxiliary channel conversion results are based on the volt-
age applied to REFIN2. This is a single-ended reference input
specified for 2.5 V operation but functional with input voltages
from 1 V to AV
DD
.
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