TS6001
improved output current accuracy, I
SET
should be at
least 10 times I
QSC
.
The circuit illustrated in Figure 5 avoids the need for
multiple op amps and well-matched resistors by
using an active integrator circuit. In this circuit, the
voltage reference’s output is used as the input signal
to the integrator. Because of op amp loop action, the
integrator adjusts its output voltage to establish the
correct relationship between the reference’s OUT
and GND terminals (=V
REF
). In other words, the
output voltage polarity of the integrator stage is
opposite that of the reference’s output voltage.
Figure 4:
A Low-power, General-Purpose Current
Source.
A Negative, Precision Voltage Reference without
Precision Resistors
When using current-output DACs, it is oftentimes
desired that the polarity of the output signal voltage
is the same as the external reference voltage. There
are two conventional techniques used to accomplish
this objective: a) inverting the full-scale DAC output
voltage or b) converting a current-output DAC into a
voltage-switching DAC. In the first technique, an op
amp and pair of precision resistors would be
required because the DAC’s output signal voltage
requires re-inversion to match the polarity of the
external reference voltage. The second technique is
a bit more involved and requires converting the
current-output DAC into a voltage-switching DAC by
driving the DAC’s VREF and IOUT terminals in
reverse. Additional components required are two
precision resistors, an op amp, and an external
voltage reference, typically a 1.25-V reference. If the
1.25-V full-scale output voltage requires scaling to a
2.5-V or a 5-V full scale, then a second op amp and
pair of precision resistors would be necessary to
perform the amplification.
To avoid the need for either re-inversion of the
current-switching DAC’s output voltage or amplifying
the voltage-switching DAC’s output voltage, it would
then be desired to apply a negative voltage
reference to the original current-switching DAC. In
general, any positive voltage reference can be
converted into a negative voltage reference using
pair of matched resistors and an op amp configured
for inverting mode operation. The disadvantage to
this approach is that the largest single source of
error in the circuit is the relative matching of the
resistors used.
Figure 5:
How to Convert a V
REF
to a –V
REF
without
Precision Resistors.
The 2200pF capacitor at the output of the TS6001 is
optional and the resistor in series with the output of
the op amp should be empirically determined based
on the amplifier choice and whether the amplifier is
required to drive a large capacitive load.
Rail-to-rail output op amps used for the integrator
stage work best in this application; however, these
types of op amps require a finite amount of
headroom (in the millivolt range) when sinking load
current. Therefore, good engineering judgment is
always recommended when selecting the most
appropriate negative supply for the circuit.
How to Use the TS6001 in a High-Input Voltage
Floating Current Source
By adopting the technique previously shown in
Figure 2, the basic floating current source circuit can
be adapted to operate at much higher supply
voltages beyond the supply voltage rating of the
TS6001-2.5 by adding a discrete n-channel JFET.
As shown in Figure 6, the JFET acts as a supply
voltage regulator since its source voltage will always
be 2.5V higher than V
SY
. The circuit minimizes
reference IC self-heating because the JFET and the
2N3904 NPN transistor carry the load current. This
circuit can operate up to +35V and is determined by
the BV
DS
breakdown voltage of the external JFET.
Page 9
RTFDS
TS6001DS r1p0
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
