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1- A voltage reference duel of duals

Part 1- A voltage reference duel of duals

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Part 1- A voltage reference duel of duals

Ying Zhou1

Sep 03 2014 02:00 AM

Comments 11

Developing a low-drift
system can be very difficult, especially with a bipolar input signal.
Applications such as bidirectional current sensing shown in Figure 1, require
the use of two well-matched, low-drift reference voltages. The first voltage,
VREF, defines the full-scale range of the ADC. A bias voltage,
VBIAS, is necessary in order to level shift the bipolar signal. It is
desirable to have VBIAS = VREF/2 so that there are equal
positive and negative swings for the ADC. I’ll discuss the three topologies for
generating two reference voltages in this three part series.

Figure
1: Low-drift bidirectional single-supply low-side current sensing
system

Using
two separate voltage references, as shown in Figure 2, provides a
straightforward approach.

Figure 2: Solution 1 (two
separate references)

For the application in
Figure 1, VREF and VBIAS are 3.0V and 1.5V,
respectively.  Table 1 lists a couple of low-drift references. Considering
drift, accuracy, and cost, the REF5030A is a
good option for the 3V reference voltage. Unfortunately, a low-drift, fixed 1.5V
reference is not readily available. In this case, you might select a 1.25V
reference, such as the LM4140B, though
this unbalances the positive and negative ranges.

Table 1: Low-drift Voltage
References

A secondary solution
is possible, given a 3V voltage reference using the voltage divider as shown in
Figure 3.

Figure 3: Solution 2
(reference + voltage divider + buffer)

Here, the drift
of VBIAS, as shown in Equation (1), comes from the drifts of the
reference (δREF), resistor divider network (δRDIV), and
buffer amplifier (δBUF).

For a comparable
low-drift solution, select resistors with 0.1% tolerance and 5ppm/°C
temperature drift. Considering the full-scale range for the amplifier is 1.5V,
the offset of the buffer amplifier is not significant. Targeting 0.1% error due
to input offset voltage and 1 ppm/°C drift error, the amplifier should
have less than 1.5 mV offset voltage and 1.5 µV/°C drift. Table 2 shows the
devices selected for this solution. For more detail on component selection,
please refer to TIPD156, a
current sensing reference design from the TI Designs Precision
library.

Table
2: components for second solution

Take a quick look at
the two solutions proposed here:

Solution 1 is a
straightforward approach, but restricted to fixed reference outputs and
VBIAS is not necessarily VREF/2.

Solution 2 uses
only one reference, so the drift of VBIAS will track with the drift
of VREF. Since the resistor can adjust accordingly, this solution
has great value when VBIAS ≠ VREF/2. On the downside,
this solution requires more components.

Figure 4
shows a third solution that uses a dual-output voltage reference (REF2030) to
offer both the VREF and VBIAS from one chip. Two
independent buffers generate the VREF and VBIAS from the
band-gap voltage. The internal resistors are sized such that VBIAS =
VREF/2. Table 3 lists the main specifications of the REF2030.

Figure
4: Solution 3 (Dual-output reference voltage- REF2030)

Table 3: REF2030 specifications

So far we’ve shown the
three topologies to generate two reference voltages. Stay tuned next week, when
we’ll compare the performance of these three solutions.

Resources:

TIPD156: Low-Drift
Bidirectional Single-Supply Low-Side Current Sensing Reference Design

Part 2: A voltage
reference dual of duels

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voltage
reference, TI
Designs - Precision, TIPD156,
REF2030,
REF
REF2030

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Comments

Bonnie
Baker - WEBENCH Design Center

Sep 04 2014 15:47 PM

Ying,

Excellent voltage reference discussion. I like that fact that you presented
3 options that can be used for this application probelm. I am looking forward
to your performance comparision.

Ying
Zhou1

Sep 08 2014 22:36 PM

Thanks, Bonnie!

Soufiane
Bendaoud

Sep 10 2014 00:33 AM

Well written, clear and concise, thank you. Consider a follow up on the
noise impact on data converters (from a voltage reference)

Ying
Zhou1

Sep 20 2014 17:17 PM

Hi Soufiane,

Thanks for your suggestion. I'll consider the idea of discussing the noise
impact on data converters. :-)

Rookie

Sep 21 2014 10:42 AM

I don't understand how the circuit in Figure 1 could function
properly

John
Handwork

Sep 21 2014 11:07 AM

Rookie,

Figure 1 can't work because it wasn't transcribed from TIPD156 properly
where you'll find a complete difference amplifier with all 4 resistors.

Ying,

Could you update Figure 1 to accurately reflect the circuit in TIPD156?
If the intent was to try to simplify the illustration, it didn't help.
:-)

Neil
Albaugh

Sep 21 2014 11:58 AM

A constant current in a fixed resistor generates a constant voltage. By
using a REF200 a voltage reference of microvolts to volts can be easily
generated. A "4th Method"?

Ying
Zhou1

Sep 21 2014 15:28 PM

Hi Rookie, John,

You are right. The INA topology I showed was not correct. I just updated
the graph. Thanks for pointing this out! :-)

Ying

Ying
Zhou1

Sep 21 2014 15:44 PM

Hi Neil,

That could be a method to consider. But here, we are targeting at a
low-drift system. The REF200 (25ppm/°C, typ) is not as low-drift as the
reference voltages we proposed here. :-)

Thanks,

Ying

M
Haglund

Sep 22 2014 02:53 AM

Interesting article of a classic challenge!

I would not mind in the upcoming article if you also mention something
about drift over time as well.

Cheers

Ying
Zhou1

Sep 22 2014 09:02 AM

Hi Matt,

Thanks for your comment. The error caused by drift over temp is discussed
in Part 2 :-)

Ying

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