AD7785
Rev. 0 | Page 26 of 32
BURNOUT CURRENTS
Burnout currents are available on Channels AIN1 and AIN2.
The burnout currents are 100 nA constant current generators,
one sourcing current from AV
DD
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 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,
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
register. The current sources work over the normal absolute
input voltage range specifications with buffers on.
EXCITATION CURRENTS
The AD7785 also contains two matched, software-configurable,
constant current sources that can be programmed to equal
10 μA, 210 μA, or 1 mA. Both source currents from the AV
DD
are directed to either the IOUT1 or IOUT2 pin of the device.
These current sources are controlled via bits in the IO register.
The configuration bits enable the current sources, direct the
current sources to IOUT1 or IOUT2, and select 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 AD7785. This biases
the negative terminal of the selected input channel to AV
DD
/2.
It is useful in thermocouple applications, because 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 to ensure that
signals close to GND or AV
DD
are converted accurately.
The bias voltage generator is controlled using the VBIAS1 and
VBIAS0 bits in conjunction with the boost bit in the configura-
tion register. The power-up time of the bias voltage generator is
dependent on the load capacitance. To accommodate higher
load capacitances, the AD7785 has a boost bit. When this bit is
set to 1, the current consumed by the bias voltage generator
increases, so that the power-up time is considerably reduced.
Figure 10 shows the power-up time when boost equals 0 and 1
for different load capacitances.
The current consumption of the AD7785 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 AD7785 has an embedded 1.17 V reference that can be
used to supply the ADC, or an external reference can be
applied. The embedded reference is a low noise, low drift
reference, the drift being 4 ppm/°C typically. For external
references, the ADC has a fully differential input capability for
the channel. The reference source for the AD7785 is selected
using the REFSEL bit 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 AV
DD
. 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 AD7785 is functional with reference voltages
from 0.1 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 is removed because the application is ratiometric. If the
AD7785 is used in a nonratiometric application, a low noise
reference should be used.
Recommended 2.5 V reference voltage sources for the AD7785
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 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 previously
(such as the 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.
RESET
The circuitry and serial interface of the AD7785 can be reset
by writing 32 consecutive 1s to the device. This resets the logic,
the digital filter, and the analog modulator while 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.
