REV. 0
AD7708/AD7718
–39–
The output code for any analog input voltage on the AD7708
can be represented as follows:
Code = (AIN × GAIN × 2
16
)/(1.024 × V
REF
)
where
AIN is the analog input voltage,
GAIN is the PGA gain, i.e., 1 on the 2.5 V range and 128 on
the 20 mV range.
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 AD7718 for any
analog input voltage can be represented as follows:
Code = 2
23
× [(AIN × GAIN/(1.024 × V
REF
)) + 1]
where
AIN is the analog input voltage,
GAIN is the PGA gain, i.e., 1 on the ±2.5 V range and 128 on
the ±20 mV range.
The output code from the AD7708 for any analog input voltage
can be represented as follows:
Code = 2
15
× [(AIN × GAIN/(1.024 × V
REF
)) + 1]
where
AIN is the analog input voltage,
GAIN is the PGA gain, i.e., 1 on the ±2.5 V range and 128 on
the ±20 mV range.
Oscillator Circuit
The AD7708/AD7718 is intended for use with a 32.768 kHz
watch crystal or ceramic resonator. A PLL internally locks onto
a multiple of this frequency to provide a stable 4.194304 MHz
clock for the ADC. The modulator sample rate is the same as
the 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 AD7708/
AD7718 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 AD7708/AD7718 has a fully differential reference input
capability. When the AD7708/AD7718 is configured in 8-channel
mode (CHCON = 0) the user has the option of selecting one of
two reference options. This allows the user to configure some
channels, for example, for ratiometric operation while others can
be configured for absolute value measurements. The REFSEL bit
in the mode register allows selection of the required reference.
If the REFSEL bit is cleared, the reference selected is REFIN1(+)
–REFIN1(–) for the active channel. If this bit is set, the refer-
ence selected is REFIN2(+) – REFIN2(–) for the active channel.
When the AD7708/AD7718 is configured in 10-channel mode
(CHCON = 1) the user has only one reference option (REFIN1).
The contents of the CHCON bit overrides the REFSEL bit. If
the ADC is configured in five fully-differential or 10 pseudo-
differential input channel mode, the REFSEL bit setting is
irrelevant as only one reference input is available.
The common-mode range for these differential inputs is from
AGND to AV
DD
. The reference inputs are unbuffered and
therefore excessive R-C source impedances will introduce gain
errors. The nominal reference voltage for specified operation,
VREF, (REFIN1(+)–REFIN1(–) or REFIN2(+)–REFIN2(–)),
is 2.5 V, but the AD7708/AD7718 is functional with reference
voltages from 1 V to AV
DD
. 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 will be removed as the
application is ratiometric. If the AD7708/AD7718 is used in a
nonratiometric application, a low noise reference should be
used. Recommended reference voltage sources for the AD7708/
AD7718 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 of having
decoupling capacitors on the REFIN(+) without introducing gain
errors in the system. Deriving the reference input voltage across
an external resistor will mean that the reference input sees a
significant external source impedance. External decoupling on
the REFIN(+) and REFIN(–) pins would not be recommended
in this type configuration.
RESET Input
The RESET input on the AD7708/AD7718 resets all the logic,
the digital filter and the analog modulator while all on-chip
registers are reset to their default state. RDY is driven high and
the AD7708/AD7718 ignores all communications to any of its
registers while the RESET input is low. When the RESET input
returns high the AD7708/AD7718 operates with its default setup
conditions and it is necessary to set up all registers and carry out
a system calibration if required after a RESET command.
Power-Down Mode
Loading 0, 0, 0 to the MD2, MD1, MD0 bits in the ADC mode
register places the ADC in device power-down mode. Device
power-down mode is the default condition for the AD7708/
AD7718 on power-up. The ADC retains the contents of all its
on-chip registers (including the data register) while in power-
down. The device power-down mode does not affect the digital
interface, but does affect the status of the RDY pin. Writing the
AD7708/AD7718 into power-down will reset the RDY line high.
Placing the part in power-down mode reduces the total current
(AI
DD
+ DI
DD
) to 31 µA max when the part is operated at 5 V
and the oscillator allowed to run during power-down mode.
With the oscillator shut down the total I
DD
is typically 9 µA.
