slyt339.pdf

Texas Instruments Incorporated

Data Acquisition

How the voltage reference affects

ADC performance, Part 2

By Miro Oljaca, Senior Applications Engineer,

and Bonnie Baker, Senior Applications Engineer

Introduction

This article is Part 2 of a three-part series that investigates

the design and performance of a voltage-reference system

for a successive-approximation register (SAR) analog-to-

digital converter (ADC). A simplified version of this system

is shown in Figure 1. When a design uses an ADC in this

system, it is critical to understand the voltage-reference

path to the converter. Part 1 (see Reference 1) examined

the fundamental operation of an ADC independent of the

voltage reference, and then analyzed the performance

characteristics that have an impact on the accuracy and

repeatability of the system. Part 2 looks at the key charac-

teristics of the voltage-reference block in Figure 1 and the

reference’s possible impact on the ADC’s performance.

Part 2 also shows how to design an appropriate external

reference for 8- to 14-bit ADCs. Part 3, which will appear

in a future issue of the Analog Applications Journal , will

investigate the impact of the voltage-reference buffer and

the capacitors that follow it, discuss how to ensure that the

amplifier is stable, and provide a reference design that is

appropriate for ADCs with 16+ bits.

Choosing the correct V REF topology

Voltage references are available in two-terminal shunt or

three-terminal series configurations (see Figure 2). Figure

2a shows a two-terminal shunt voltage reference, in which

the entire IC chip of

the shunt reference

operates in parallel to

its load. With a shunt

voltage reference, an

input voltage is

applied to the resistor

that is connected to

the cathode. The

typical initial voltage

accuracy of this

device can be as low

as 0.5% or range up

to 5%, with a temper-

ature coefficient of

approximately 50 to

100 µV/°C. The shunt

voltage reference can

be used to create

positive, negative, or

floating reference

voltages.

Figure 1. Voltage-reference system for SAR ADC

V IN

Voltage

Reference

V REF

D OUT

AIN

ADC

The three-terminal series voltage reference (Figure 2b)

operates in series with its load. An internal bandgap volt-

age, in combination with an internal amplifier, creates the

output voltage of this reference. The series voltage refer-

ence produces an output voltage between the output and

ground while providing the appropriate output current to

Figure 2. Voltage-reference configurations

V IN

Cathode

Output

Amplifier

–

V OUT

Ref

1.2 V

Load

–

Bandgap

V REF

Series

Reference

Shunt

Reference

Anode

(a) Two-terminal shunt

(b) Three-terminal series

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the external load. As the load current increases or

decreases, the series reference maintains the voltage

at V OUT .

The typical initial voltage accuracy of a series-reference

device can be as low as 0.05% or range up to 0.5%, with

temperature coefficients as low as 2.5 ppm/°C. Because

of the series reference’s superior initial output voltage and

overtemperature performance, this type of device would

be used to drive the reference pins of precision ADCs.

Beyond 8 or 14 resolution bits, where the size of the least

significant bit (LSB) is respectively 0.4% and 0.006%, an

external series voltage reference ensures that the intended

precision of the converter can be achieved.

Another common application for series voltage refer-

ences is sensor conditioning. In particular, a series voltage

reference is useful in bridge-sensor applications as well as

applications that have thermocouples, thermopiles, and

pH sensors.

The initial accuracy of the series voltage reference in an

ADC application (as in Figure 1) provides the general ref-

erence for the conversion process. Any initial inaccuracy of

the output voltage can be calibrated in hardware or soft-

ware. Additionally, changes in the accuracy of the voltage-

reference output can be a consequence

of the temperature coefficient, the line

regulation, the load regulation, or long-

term drift. The series voltage reference

provides better performance in all of

these categories.

Understanding reference-

voltage noise

From Part 1 of this series it can be con-

cluded that the ADC has only one func-

tion. That function is to compare an input

voltage to a reference voltage, or to create

an output code based on an input signal

and reference voltage. Part 1 presented

diagrams and formulas that describe the

basic transfer function of the ADC along

with the device’s noise characteristics. The

typical transfer function of an ideal ADC,

shown here in Figure 3, was described as

Figure 3. An ideal, 3-bit ADC transfer function

111

110

101

100

011

010

001

000

Full-Scale Range

AIN

Negative

Full Scale

Positive

Full Scale

Figure 4. Total noise at ADC output as a function of ADC

input voltage

ADC + Reference Noise

Reference

Noise

ADC Internal Noise

0.5

1.5

2.5

3.5

4.5

AIN

=× 2

Code

(1)

REF

where “Code” is the ADC output code in decimal form, V IN

is the analog input voltage to the ADC, n is the number of

ADC output bits, and V REF is the analog value of the ref-

erence voltage to the ADC. This formula shows that any

initial error or noise in the reference voltage translates to

a gain error in the code output of the ADC.

If several points from the ADC’s negative full-scale input

to its positive full-scale input are measured, it becomes

clear that the contribution of the reference noise is a func-

tion of the ADC input voltage. To evaluate the voltage-

reference noise as well as the overall noise, it is necessary

to measure the noise close to both the negative full scale

and the positive full scale. Figure 4 shows the results of

measuring the reference noise and the ADC noise in a

system. These results show that the overall noise is not

constant but linearly dependent on the ADC’s analog input

voltage. When this type of system is designed, it is impor-

tant to keep the reference noise lower than the ADC’s

internal noise.

Both reference topologies in Figure 2 generate compa-

rable noise over frequency. The voltage noise in series

voltage references comes mainly from the bandgap and

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the output amplifier. Both of these elements gener-

ate noise in the 1/f region and the broadband region

(see Figure 5).

Noise in the voltage reference’s 1/f region

In the data sheets of most series-reference devices,

the specification for output-voltage noise is over the

frequency range of 0.1 to 10 Hz, which encompasses

the 1/f region in Figure 5. Noise in the 1/f region,

often called “pink noise,” is replaced in the higher

frequency domain by the broadband noise.

Noise in the voltage reference’s broadband region

Some manufacturers include specifications for the

voltage reference’s output noise density. This type of

specification is usually for noise in the broadband

region, such as the noise density at 10 kHz. Broad-

band noise, which is present over the higher wide-

band frequencies, is also known as “white noise” or

“thermal noise.”

An added low-pass filter with an extremely low

corner frequency will reduce the broadband noise at the

output of the reference. This filter is designed with a

capacitor, the equivalent series resistance (ESR) of the

capacitor, and the open-loop output impedance of the

reference output amplifier (see Figure 6).

Table 1 shows the noise measured from the Texas

Instruments REF5040 for different frequency bandwidths

as well as for different external-capacitor values and

types. These measurements demonstrate that ceramic

capacitors with a low ESR of about 0.1 Ω have a tendency

to increase noise compared to tantalum capacitors with a

standard ESR of about 1.5 Ω. This tendency is the result of

stability problems and the gain peaking of the reference’s

output amplifier.

As mentioned earlier, the two sources of noise in the

reference voltage are the internal output amplifier and

the bandgap. The internal schematic of the REF5040 in

Figure 7 shows that the TRIM pin provides direct access

to the bandgap. An external capacitor can be added to the

TRIM pin to create a low-pass filter. This filter provides a

Figure 5. Example voltage-noise regions in the

frequency domain

100

1/f

Region

Broadband

Region

Low-Pass

Filter

100

1000

10000

Frequency (Hz)

Figure 6. Low-pass filter between series

voltage reference and ADC

V IN

Voltage

Reference

Output

Amplifier

–

R O

Bandgap

Reference

V OUT

ESR

C L1

V REF

D OUT

AIN

ADC

Figure 7. Using TRIM pin to filter REF5040

bandgap noise

Table 1. Noise measured from REF5040 for different bandwidths

and capacitor values and types

MEASURED NOISE (µV RMS )

FOR FOUR BANDWIDTHS

REF5040

CAPACITOR

22 kHz

(Low-Pass

5-Pole)

30 kHz

(Low-Pass

3-Pole)

80 kHz

(Low-Pass

3-Pole)

>500 kHz

–

GND

0.8

1.8

4.9

V OUT

10 k 

1 µF (tantalum)

37.8

41.7

53.7

9017

1.2 V

2.2 µF (ceramic)

41.7

46.2

55.1

60.8

TRIM

10 µF (tantalum)

33.4

35.2

38.5

1 k 

10 µF (ceramic)

37.1

37.2

37.8

39.1

1 µF

20 µF (ceramic)

33.1

33.2

34.5

47 µF (tantalum)

23.2

23.8

24.1

26.5

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bandgap broadband attenuation of approximately –21 dB.

For example, a small 1-µF capacitor adds a pole at 14.5 Hz

and a zero at 160 Hz. If more filtering is needed, a larger-

value capacitor can be used in place of the 1-µF capacitor.

For instance, a 10-µF capacitor will generate a 3-dB corner

frequency of 1.45 Hz. This low-pass filter will lower the

bandgap noise. Attaching a 1-µF capacitor to the TRIM pin

of the REF5040 will lower the total output RMS noise by a

factor of 2.5.

Conclusion

Figure 8 shows a complete circuit diagram for a reference

system configured with an 8- to 14-bit converter. The accu-

racy of the voltage reference in this system is important;

however, any initial inaccuracy can be calibrated with

hardwareorsoftware.Ontheotherhand,eliminatingor

reducing reference noise will require a degree of charac-

terization and hardware-filtering techniques. Part 3 of this

article series will explore the proper filtering for the

broadband region.

Part 3 will also investigate and explain how to design a

reference circuit that is appropriate for converters with

16+ bits. The impact of the voltage-reference buffer and

its following amplifier/resistor/capacitor network will be

analyzed. With the measurements that follow the final sys-

tem tuning, the assumptions and conclusions of this article

series will be compared to the real world.

References

For more information related to this article, you can down-

load an Acrobat ® Reader ® file at www-s.ti.com/sc/techlit/

litnumber and replace “ litnumber ” with the TI Lit. # for

the materials listed below.

Document Title

TI Lit. #

1.BonnieBakerandMiroOljaca,“Howthe

Voltage Reference Affects ADC Performance,

Part 1,” Analog Applications Journal

(2Q 2009) ............................... slyt331

2. Bonnie Baker, “A Glossary of Analog-to-

Digital Specifications and Performance

Characteristics,” Application Report ......... sbaa147

3.TimGreen.Operationalamplifierstability,

Parts 3, 6, and 7. EN-Genius Network:

analogZONE: acquisitionZONE [Online].

Available: http://www.analogzone.com/

acqt 0000 .pdf (Replace “ 0000 ” with “0307 ” for

Part 3, “0704 ” for Part 6, or “0529” for Part 7.)

—

4.BonnieC.BakerandMiroOljaca.(2007,

June 7). External components improve

SAR-ADC accuracy. EDN [Online].Available:

http://www.edn.com/contents/images/

6447231.pdf

—

5.Wm.P.(Bill)Klein,MiroOljaca,andPete

Goad. (2007). Improved voltage reference

circuits maximize converter performance.

AnalogeLab™Webinar[Online].Available:

http://dataconverter.ti.com (Scroll down to

“Videos” under “Analog eLab™ Design

Support” and select webinar title.)

Figure 8. Voltage-reference circuit for 8- to

14-bit converters

—

6. Art Kay. Analysis and measurement of

intrinsic noise in op amp circuits, Part I.

EN-Genius Network: analogZONE:

audiovideoZONE [Online].Available:

http://www.en-genius.net/includes/files/

avt_090406.pdf

V IN

Voltage

Reference

–

R O

—

10 k 

Bandgap

Reference

V OUT

10 µF

( Tantalum )

Related Web sites

dataconverter.ti.com

www.ti.com/sc/device/REF5040

1 k 

TRIM

1µF

V REF

D OUT

AIN

ADC

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