Developing a =
low-drift=20
system can be very difficult, especially with a bipolar input signal.=20
Applications such as bidirectional current sensing shown in Figure 1, =
require=20
the use of two well-matched, low-drift reference voltages. The first =
voltage,=20
VREF, defines the full-scale range of the ADC. A bias =
voltage,=20
VBIAS, is necessary in order to level shift the bipolar =
signal. It is=20
desirable to have VBIAS =3D VREF/2 so that there =
are equal=20
positive and negative swings for the ADC. I=E2=80=99ll discuss the three =
topologies for=20
generating two reference voltages in this three part series.
Figure=20
1: Low-drift bidirectional single-supply low-side current sensing=20
system
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
Using=20
two separate voltage references, as shown in Figure 2, provides a=20
straightforward approach.
Figure 2: Solution =
1 (two=20
separate references)
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
For the =
application in=20
Figure 1, VREF and VBIAS are 3.0V and 1.5V,=20
respectively. Table 1 lists a couple of low-drift references. =
Considering=20
drift, accuracy, and cost, the REF5030A is a=20
good option for the 3V reference voltage. Unfortunately, a low-drift, =
fixed 1.5V=20
reference is not readily available. In this case, you might select a =
1.25V=20
reference, such as the LM4140B, though=20
this unbalances the positive and negative ranges.
Table 1: Low-drift =
Voltage=20
References
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
A secondary =
solution=20
is possible, given a 3V voltage reference using the voltage divider as =
shown in=20
Figure 3.
Figure 3: Solution =
2=20
(reference + voltage divider + buffer)
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
Here, =
the drift=20
of VBIAS, as shown in Equation (1), comes from the drifts of =
the=20
reference (=CE=B4REF), resistor divider network =
(=CE=B4RDIV), and=20
buffer amplifier (=CE=B4BUF).
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
For a =
comparable=20
low-drift solution, select resistors with 0.1% tolerance and =
5ppm/=C2=B0C=20
temperature drift. Considering the full-scale range for the amplifier is =
1.5V,=20
the offset of the buffer amplifier is not significant. Targeting 0.1% =
error due=20
to input offset voltage and 1 ppm/=C2=B0C drift error, the =
amplifier should=20
have less than 1.5 mV offset voltage and 1.5 =C2=B5V/=C2=B0C drift. =
Table 2 shows the=20
devices selected for this solution. For more detail on component =
selection,=20
please refer to TIPD156, a=20
current sensing reference design from the TI Designs Precision=20
library.
Table=20
2: components for second solution
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
Take a quick =
look at=20
the two solutions proposed here:
- Solution =
1 is a=20
straightforward approach, but restricted to fixed reference outputs =
and=20
VBIAS is not necessarily VREF/2.
- Solution =
2 uses=20
only one reference, so the drift of VBIAS will track with =
the drift=20
of VREF. Since the resistor can adjust accordingly, this =
solution=20
has great value when VBIAS =E2=89=A0 VREF/2. On =
the downside,=20
this solution requires more components.
Figure 4=20
shows a third solution that uses a dual-output voltage reference (REF2030) to=20
offer both the VREF and VBIAS from one chip. Two=20
independent buffers generate the VREF and VBIAS =
from the=20
band-gap voltage. The internal resistors are sized such that VBIAS =
=3D=20
VREF/2. Table 3 lists the main specifications of the REF2030.
Figure=20
4: Solution 3 (Dual-output reference voltage- REF2030)
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
Table 3: REF2030 specifications
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
So far =
we=E2=80=99ve shown the=20
three topologies to generate two reference voltages. Stay tuned next =
week, when=20
we=E2=80=99ll compare the performance of these three solutions. =
Resources: