AD5748 Datasheet, PDF (24/32 Pages) Analog Devices – Industrial Current/Voltage Output Driver with Programmable Ranges

AD5748
CURRENT OUTPUT ARCHITECTURE
The voltage input from the analog input VIN pin (0 V to 4.096 V)
is either converted to a current (see Figure 51), which is then
mirrored to the supply rail so that the application simply sees
a current source output with respect to an internal reference
voltage, or buffered and scaled to output a software-selectable
unipolar or bipolar voltage range (see Figure 52). The reference
is used to provide internal offsets for range and gain scaling.
The selectable output range is programmable through the
digital interface.
AVDD
VIN
VREF
RANGEDECODE
FROM INTERFACE
IOUT
RANGE
SCALING
R2
R3
T1
A1
T2
A2
IOUT
RSET
VIN
(0V TO 4.096V)
VREF
Figure 51. Current Output Configuration
RANGE DECODE
FROM INTERFACE
VOUT RANGE
SCALING
VOUT
SHORT FAULT
VSENSE+
VOUT
Figure 52. Voltage Output
VSENSE–
DRIVING INDUCTIVE LOADS
When driving inductive or poorly defined loads, connect a 0.01 μF
capacitor between IOUT and GND. This ensures stability with
loads beyond 50 mH. There is no maximum capacitance limit.
The capacitive component of the load may cause slower settling.
Voltage Output Amplifier
The voltage output amplifier is capable of generating both
unipolar and bipolar output voltages. It is capable of driving a
load of 1 kΩ in parallel with 1.2 μF (with an external compensa-
tion capacitor on the COMP1 and COMP2 pins). The source
and sink capabilities of the output amplifier can be seen in
Figure 16. The slew rate is 2 V/μs.
Internal to the device, there is a 2.5 MΩ resistor connected
between the VOUT and VSENSE+ pins and similarly between
the VSENSE− pin and the internal device ground. Should a
fault condition occur, these resistors act to protect the AD5748
by ensuring that the amplifier loop is closed so that the part
does not enter into an open-loop condition.
The VSENSE− pin can work in a common-mode range of ±3 V
with respect to the remote load ground point.
The current and voltage are output on separate pins and cannot
be output simultaneously. This allows the user to tie both the
current and voltage output pins together and configure the end
system as a single-channel output.
Driving Large Capacitive Loads
The voltage output amplifier is capable of driving capacitive loads
of up to 1 μF with the addition of a nonpolarized compensation
capacitor between the COMP1 and COMP2 pins.
Without the compensation capacitor, up to 20 nF capacitive loads
can be driven. Care should be taken to choose an appropriate
value for the CCOMP capacitor. This capacitor, while allowing the
AD5748 to drive higher capacitive loads and reduce overshoot,
increases the settling time of the part and, therefore, affects the
bandwidth of the system. Considered values of this capacitor
should be in the range 100 pF to 4 nF depending on the trade-
off required between settling time, overshoot, and bandwidth.
POWER-ON STATE OF THE AD5748
On power-up, the AD5748 senses whether hardware or
software mode is loaded and sets the power-up conditions
accordingly.
In software SPI mode, the power-up state of the output is depen-
dent on the state of the CLEAR pin. If the CLEAR pin is pulled
high, then the part powers up, driving an active 0 V on the
output. If the CLEAR pin is pulled low, then the part powers up
with the voltage output channel in tristate mode. In both cases, the
current output channel powers up in a tristate condition (0 mA).
This allows the voltage and current outputs to be connected
together if desired.
To put the part into normal operation, the user must set the
OUTEN bit in the control register to enable the output and, in
the same write, set the output range configuration using the R3
to R0 range bits. If the CLEAR pin is still high (active) during
this write, the part automatically clears to its normal clear state
as defined by the programmed range and by the CLRSEL pin or
CLRSEL bit (see the Asynchronous Clear (CLEAR) section for
more details). The CLEAR pin must be taken low to operate the
part in normal mode.
The CLEAR pin is typically driven directly from a microcontroller.
In cases where the power supply for the AD5748 supply may be
independent of the microcontroller power supply, the user can
connect a weak pull-up resistor to DVCC or a pull-down resistor
to ground to ensure that the correct power-up condition is
achieved independent of the microcontroller. A 10 kΩ pull-
up/pull-down resistor on the CLEAR pin should be sufficient
for most applications.
If hardware mode is selected, the part powers up to the condi-
tions defined by the R3 to R0 range bits and the status of the
OUTEN or CLEAR pin. It is recommended to keep the output
disabled when powering up the part in hardware mode.
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