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2 - A voltage reference duel of duals

Part 2 - A voltage reference duel of duals

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Part 2 - A voltage reference duel of duals

Ying Zhou1

Sep 05 2014 14:08 PM

Comments 1

In
my previous post, we talked about three topologies for
generating two high-accuracy voltage reference outputs. Today,
we’ll compare the performance of these solutions from three perspectives: total
error, drift tracking and matching between outputs.

Total
error

Equation
(1) converts specifications given as a percentage (%) to parts per million
(ppm).

(Equation
1)

The Total
Error performance metric of each voltage output depends on its initial accuracy
and drift over the operating temperature range, as given in Equation
(2).

(Equation
2)

In
Solution 1, as no typical drift for the LM4140B is given in the datasheet, we use the
maximum drift specification, over a 70ºC temperature range for calculation. In
Solution 2, the bias voltage (VBIAS) is generated from REF5030A,
resistor network and a buffer. Therefore, the initial accuracy and drift can be
expressed as the RSS of these three error sources, as given in Equation (1) in
Part 1. Since the REF2030 and REF5030A use the
box method to
determine drift, the temperature range for calculations is the entire operating
range, or 165ºC.

Table 1
shows that while the performance of VREF in Solution 1 is the same as
Solution 2, its VBIAS output has considerably more error. Note that
the error for VBIAS in Solution 2 includes the error from
VREF. With high initial accuracy and low temperature drift on both
outputs, Solution 3 has the lowest error of the three solutions.

Table
1: Comparison of error contributions for each output voltage

Drift
tracking and matching

Another
important specification for this dual output system is drift tracking, which
describes the accuracy matching between two voltages over a particular
temperature range, as given by Equation (3). Figure 1 shows the typical drift
tracking performance of the REF2030.

(Equation
3)

Figure
1: VREF and VBIAS tracking vs. temperature

Since we
applied two independent voltage references in Solution 1, theoretically the two
references may not directly track each other, so the tracking is the RSS value
of their maximum temperature drifts (11 ppm/ºC). As the LM4140B
is only specified from 0°C to 70ºC, this drift tracking only applies for this
temperature range.

In
Solution 2, since error in VREF is common to both outputs, the drift
tracking (δTracking) between VREF and VBIAS
only depends on drift from resistor network (δRES) and buffer
(δBUF), as given by Equation (4).

(Equation
4)

Given the
initial accuracy error, we can also calculate the matching (at 25ºC) of the
outputs in terms of RSS, as shown in Equation (5).

Table 2
shows a comparison summary. Note that the drift tracking and output matching in
Solution 2 heavily depend on the precision of the resistors. While the tracking
of the two outputs in Solution 2 is slightly better, the matching of the outputs
is much worse than that in Solution 3. Actually, Solution 3 is about 900 ppm
better. This means, with only a 2 ppm/°C difference in drift, it will take a
450°C of temperature shift before Solution 2 becomes the more accurate solution.
For details on calculation, please refer to data.xlsx.

Table
2: Comparison of output matching and drift tracking

From our
calculations we know Solution 3 has the best overall performance in most cases.
However, in reality, analog engineers have to consider more than performance.
Stay tuned for next week, when we compares the three solutions with respect to
PCB space and cost.

Related
resources:

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

Voltage Reference
Selection Basics

Part 1: A voltage
reference duel of duals

Attachment: Data.xlsx

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$post.CommentCount)

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

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Comments

Bonnie
Baker - WEBENCH Design Center

Sep 10 2014 14:31 PM

Ying,

Excellent article. Clear and quickly to the point. I look forward to the
next installment.

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