Cirrus Logic AN31 User Manual

AN31

Application Note

A Collection of Brid ge Transducer Digitizer Cir cuits

Jerome Johnston

Introduction

Bridge transducers are common in instrumentation. This application note illustrates some bridge transducer digitizer circuits which use the CS5504/5/6/ 7/8/9 A/D converters and the CS5516/20 A/D co nverters.

The CS5504/5/6/7/8 converters can be operated with a variety of power supply arrangements; including operating from a single +5 V supply; operating from +5 and -5 analog supplies with +3.3 V or +5 V on the digital supply; or operating with an analog supply from +5 to +11 V and a digi tal supply of +5 V.

The CS5509 can oper ate with +5 V on its analog and digital supplies; or with +5 V analog and +3.3 V digita l.

The CS5516 and CS5520 are A/D converters optimized for bridge transducer applications and are designed to operate from +5 and -5 V supplies. Several circuits which utilize these ADCs will be presented.

The applicatio n note is divi ded into two sections:

1. DC-excited bridge circuits.

2. AC-excited bridge circuits with a discu ssion of the benefits of AC excitation.

Bridge Tran sducers

Bridge transducers are manufactured with various technologies. The strain-sensing elements which make up the bridge may be made of diffused silicon, bonded silicon bars, deposited thin film, or bonded foil materials. The choice of technology will determine the performance of the transducer, including the sensitivity, the linearity, and the thermal stability. Si licon-based gages hav e good linearity with sensitivities between 3 mV/V and 20 mV/V, but tend to exhibit more drift as temperature changes. Metal foil or thin film gages have good linearity with sensitivities between 1 mV/V and 4 mV/V. Precision bridge transducers include some type of temperature compensation as part of the bridge .

Most bridge circuits are excited with a dc voltage, 10 v olts bei ng very common . With 10 V excitation, the full scale signa ls from the various transducers, can be as low as 10 mV to as high as several hundred millivolts. When digitizing these signals to high resolution (for discussion in this application note, high resolution means greater than 10,000 counts), one count can represent a very small voltag e. It can be diffi cult to amplify and digitize these low level bridge transducer signals. Measurement performance can be hindered by such things as amplifier

Crystal Semiconductor Corporation

P.O. Box 17847, Austin, TX 78760 (512) 445-7222 FAX 445-7581 http://www.crysta l.com

JAN ’95

AN31REV3

Bridge Transducer Digitizer Circuits

offset drift, amplifier noise (both thermal and 1/f), amplifier finite open loop gain, and parasitic thermocouples. Parasitic thermocouples are introduced any time two dissimilar metals are connected. For example, using tin-lead solder to solder a wire to a copper PC trace can introduce an unwanted thermocouple junction

which changes as much as 3 µV/°C when

subjected t o temperat ure gradient s.

This application note will introduce some A/D converter circuits which illustrate a number of application ideas to the design engineer who uses bridge transducers. In the AC-excited bridge section, a number of design ideas will b e introduced which offer very good solutions to some of the problems encountered in low level bridge measurement.

5.23k

350350

100 pF

350 350

40.2k

Optional

Zero Trim

3 mV/V Transducer

LTC1051

0.1

6 5

0.47

0.47 7

DC-EXCITED BRIDGE CIRCUITS

CS5507,8,9 Bridg e Transducer Operating From a Single +5 V Supply, or with the Analog Supply at +5 V and Digital Supply at +3.3 V

Figure 1 illustrates the low cost CS5509 16-bit converter operating from +5 V. The A/D can operate in either unipolar or bipolar mode and yields 20 c onversions/second wh en running from a low cost 32.76 8 kHz crystal. When operated at

32.768 kHz the digital filter in the converter notches out 50 and 6 0 Hz line interfere nce.

The LTC1051 dual chopper amplifier is used as the bridge amplifier. Bandwidth is limited to

about 3.8 Hz b y the 100k and 0.47 µF feedback

+3.3 V

500

10k

100k

931

100k

100

0.1

Optional

Gain Trim

≈

3.25 V

0.1

VA+ VD+

VREF+

VREF-

AIN+

AIN-

Regulator

11 13

XIN

XOUT

CAL

CONV

CS5509

16 bits

DRDY

SDATA

BP/UP

SCLK

+5V or +3.3V

VD+ can be

32.768kHz

Microcontroller

0.1

System

20 Conversions/sec.

GND

Figure 1. CS5507,8,9 Bridge Transducer Operating from a Single +5 V Supply, or with the Analog Supply at +5 V

and Digital Supply at +3.3 V.

2 AN31REV3

Bridge Transducer Digitizer Circuits

elements of the amplifier stage. Note that an instrumentation amplifier is not needed because the A/D input is fully differential. The dual amplifier functions as a differential in, differential out amplifier. The circuit yields about 9000 noise-free counts when measuring unipolar s ignals. Averag ing 10 sample s increases this to about 28,500 noise-free counts. "Noise-free counts" means full scale signal divided by six times the rms noise. Noise-free counts is good figure of merit for comparing A/D converters used in dc measurement applications. There is more discussion on this topic at the e nd of the app lication no te.

The circuit illustrated uses a 3 mV/V transducer excited with +5 V for a full scale transducer output of 15 mV. The transducer output is amplified with a gain of about 216 to yield

3.25 V full scale. A dual stage amplifier, as shown in Figure 2 may be preferred to minimiz e errors due to limited loop gain. The A/D is operated in bipolar mode to achieve more

µV/LSB. The reference voltage for the converter is derived from th e +5 V excitation voltage. The measurement remains ratiometric should the +5 V excitatio n change.

Figure 1 includes potentiometers for offset and gain adjustment , as do a numbe r of other circuits in this application note. Many system designers prefer to eliminate potentiometers and do all offset and gain correction in software. To

10k

x216

4.32k

20k

Figure 2. Dual Stage Amplifier

achieve this in some of the circuits may require changes to gain stages or voltage references , but potentiometers are shown for all the engineers who are more comfortable with screwdrivers than software.

The CS5509 in Figure 1 can run as fast as 200 conversions per second if operated with a 330kHz external clock. Figure 3 shows an RC gate oscillator which can produce stable frequencies, or a CMOS 555 timer can be used. The gate oscillator ca n be operated from either a +5 or +3.3 V supply and maintains fairly good frequency sta bility ove r temperatur e.

+3.3 to +5

74HC04

0.1

f 162kHz 200kHz

output

330kHz

f ≈

2 (R + R ) C

10k

8.2k 5k

1.44

1 2

3.4k

2.7k

1.6k

2 1

330pF 330pF 330pF

≈

Figure 3. Temp erature -Stabl e Gate Oscill ator fo r +5 or +3 .3 Vol ts.

AN31REV3 3

Bridge Transducer Digitizer Circuits

All of the converters (CS5504-09) can be operated with a single +5 V supply. All of the converters can also be operated with +5 V analog supply and +3.3 V on the digital supply. If this dual supply arrangement is used, the digital supply should be derived from the analog supply to ensure proper operation. Under all conditions, including start-up, th e voltage on the VA+ pin must be the more positive than any other pin on the device to ensure proper substrate bias ing of the c hip.

CS5507/8 with +10 V Analog Suppl y and +5 V Digital Supply

It is common for many weigh scales to be operated from batteries with a 12 V

+10

Optional

350 Ω Bridge

2mV/V

3.57k

Optional

Fine Offset Adjust

Resistor

200k

Coarse

200k

Offset

100 Conversions/sec

20k

10k

x332

+10

LT1007

0.047

+10

LT1007

Gain Trim

100

75k

453

75k

100

automotive-type battery being common. The CS5504/5/6/7/8 devices can be operated with higher supply voltage on the analog portion of the chip than on the digital portion (Note: the CS5509 is an e xception and is specified with an analog supply of +5 V onl y). The analog supply (VA+) must always be the most positive voltage on the chip to ensure proper operation. Figure 4 illustrates the CS5507 operating from +10 V on the analog an d +5 V on the digit al. The bridge is excited wit h the +10 V and resistors are used to divide this excitation supply to obtain a ratiometric vol tage reference of abou t 3.33 V for the converter. The circuit is designed to operate with the A/D in bipolar mode to yield more

µV/count. Th e A/D is set-up for an inpu t span of ± 3.33 V. A 200k pull down resistor forces a

+5Volt

Regulator

0.33

≈

3.33 V

0.047

VA+ VD+

VREF+

VREF-

AIN+

CS5507 CS5508

16 or

20 bits

AIN-

VREFOUT VA- DGND

DRDY

SCLK

SDATA

CONV

M/SLP

BP/UP

CAL

XIN

0.1

System

Microcontroller

162kHz

Figure 4. CS5507/8 with +10V Analog Supply and +5 V Digital Supply.

4 AN31REV3

Bridge Transducer Digitizer Circuits

negative offset into the amplifier and the zero trim is used to finely ad just this offset . With zero weight on the scale, the zero trim is adjusted to yield -30,000 co unts if the CS5507 1 6-bit A/D is used or to -500,000 counts if a 20-bit CS5508 is used. With full scale we ight on the scale the gai n trim is adjusted for +30,000 counts in the CS5507 or +500,000 counts in the CS5508 (Note that the CS5507 and CS5508 are pin compatible). This leaves some counts for both zero underflow and for overrange . The amplifier components set the bandwidth to 45 Hz. With the 45 Hz bandwidth, the circuit exhibits about 50,000 noise-free counts. With an external 162 kHz clock, the converter can operate at 100

7.5k

0.47

2 mV/V

350 350

-5

20 Conversions/sec

7.5k

350350

2mV/V Transducer

x100

INA131

-5

Bridge Amp

0.1

2.4k

0.1

conversions per second. If 20 conversion words from the CS5508 are averaged, the circuit will yield more than 200,000 noise-free counts. A limitation of this circuit is that the bipolar amplifiers can exhibit significant offset drift as the temperature changes. There are several circuits in this application note which will show how to overcome offset drift.

CS5505/6 Operating Fro m ± 5 V Supplies

The CS5504/5/6 /7/8 converte rs (not the CS550 9) can be operated wit h ±5 V on the analog sectio n

of the converter, and with either + 5 V or +3.3 V on the digita l section.

VD+

XIN

0.1

32.768kHz

1916

System

Microcontroller

≈

0.1

2.5 V

0.1

VA+

VREF+

VREF-

CS5505 CS5506

AIN1+

AIN2+

AIN3+

AIN4+

AIN-

-5

20 bits

VREFOUTVA- DGND

XOUT

CAL

CONV

16 or

DRDY

SDATA

BP/UP

SCLK

M/SLP

Figure 5. CS5505/6 Operating from ± 5V Supplies.

AN31REV3 5

Bridge Transducer Digitizer Circuits

Figure 5 illustrates an ap plication which uses an instrumentation a mplifier to ampli fy and convert the differential bridge signal to a ground-referenced signal for the converter. Full scale for the converter is set by the divider resistors which determine the voltage reference input to the VR EF+/- pins of the converter. The reference voltage in the figure is set to 2.5 V. The bridge sensitivity is 2 mV/V so the full

100k

TP0610L

0.1

scale bridge output is 20 mV. This is amplified by the 100 gain of the instrumentation amplifier to obtain 2 .0 V into the conv erter. The converter can be operated in either unipolar or bipolar mode. Up to four load cells, each with its own amplifier, can be input to the CS5506. The measurement assumes the voltage reference will remain ratiom etric acros s all four l oad cells .

CS5507 Switched -Bridge Lo w-power Digitizer with +10 V Excitation

Some applications call for reduced operating power. One method of significantly redu cing the power consumption is to apply the supply voltage to the bridge transducer only when a

+10

10k

500

VA+

VREF+

VREF-

+5V Regulator

VD+

XIN

0.1

Optional

Gain

Trim

3.33 V

≈

0.1

162 kHz

OSC

CAL

CONV

BP/UP

DRDY

SDATA

SCLK

M/SLP

1613

System

Microcontroller

2N7000

Q1, Q2 Siliconix

100k

350350

100pF

350

2 mV/V Transducer

0.015

LT1013

0.015

6 5

1 Conversion = 20 msec

226

18.7k

x166

18.7k

100

0.047

AIN+

CS5507 16 bits

AIN-

VREFOUTVA- DGND

Figure 6. CS5 507 S witched- Bridg e Low -power Dig itizer wit h +10 Volt Excita tion .

6 AN31REV3

Bridge Transducer Digitizer Circuits

measurement is required. Figure 6 illustrates an example circuit in which the power to the bridg e transducer is switched on only when a measurement is desired.

The circuit as shown is optimized for a +10 V analog supply. The circuit can be modified (optimized) to operate from any analog supply from 11 V to 6.5 V (assuming the +5 V regulator needs 1.5 V of input/output differential) by changing the resistor values which determine the voltage reference to the converter and by changing the gain resistors in the amplifier to compensate for the cha nge in the bridge output signal. The circuit shown illustrates a 2 mV/V transducer outputting 20 mV full-scale. A g ain of 166 ampl ifies this to

3.32 V into th e A/D. The full-scale of the A/D is set at 3.33 V by dividing down the excitation voltage.

In the power arrangement shown, the CS5507 A/D uses about 4 mW. The con verter is clocked from an external gate oscillator clock (162 kHz) to yield a conversion time of 10 msec. When power is applied to the bridge, a delay must occur to allow the signal to settle before a valid conversion can be pe rformed. Settling time to 16 bits after power is applied to the bridge takes about 3.3 msec. The microcontroller can use an internal ti mer to time about 4 msec. to allow for the delay or the microcontroller can perform a dummy conversion in the converter to allow for settling time. When the dummy conversion is finished (10 msec. later) the conversion data is discarded and a second conversion is then performed to make a valid measurement. After the second conversion is complete (

DRDY falls the second time) power to the bridge is deactivated and the conversion word is clocked

out of the conv erter’s serial port.

Power consumed by the transducer dominates the power dissipated in the circuit. Average power consumption in the bridge c an be reduced by a factor o f at leas t fifty (<6 mW) if the b ridge

is powered for only 20 msec. for a reading each second. If even lower off power is desired, the supply to the LT1013 ca n also be swit ched along with the bridge excitation.

CS5509 Switched -Bridge Lo w-power Digitizer with +5 V Excitation

The circuit in Figure 7 is similar to t he previous one, but op erates from a sing le +5 V. The ci rcuit shows a load cell with 3 mV/V sensitivity. A 2 mV/V transduce r can be use d if additional gain is added; or the voltage reference into the converter can be lowered to 1.67 V with some minor increase in noise. Average power consumption in the load cel l is only 1.5 mW for one reading per se cond.

CS5516/CS5520 Using DC B ridge Excitation

The CS5516 (16-bit) and CS5520 (20-bit) A/D converters are designed for bridge measurement applications. They include an instrumentation amplifier with X25 gain, a PGA (programmable gain amplifier) with gains of 1, 2, 4, and 8, and a four bit DAC which can trim out offset up to

± 200% of the full scale signal magnitude. The input span can be adjusted by changing either the magnitude of the voltage at the VREF pins of the converter or by changing t he PGA gain.

In the circuit shown in Figure 8, the bridge is excited with ± 5 v olts. Resis tors R1, R2 , and R3

divide the excitation voltage to give a 2.5 V reference signal into the VREF pins. The input span at the AIN pins of the converter is determined by dividing the voltage at the VREF pins by the PGA gain and the X25 instrumentation amplifier gain. For example, with 2.5 V into the VREF pins, and the PGA set to a gain o f 8, the input span at the AIN pins is

2.5/(8 X 25) = 12.5 mV in unipolar mode or ± 12.5 mV in bipolar mode. Th e converter offers

several calibration features to remove offset and to calibrate the gain slope. The input span of

AN31REV3 7

Bridge Transducer Digitizer Circuits

0.1

10k

VD+

VA+

OSC

162 kHz

XIN

VREF+

Gain

Optional

500

VREF-

Trim

10k

0.1

CAL

CONV

AIN+

100

System

Microcontroller

18.7k

0.015

DRDY

BP/UP

CS5509

16 bits

0.047

X 166

226

SCLK

SDATA

GND

AIN-

18.7k

0.015

100

100k

350350

100 pF

LT1013

350

3 mV/V

1 Conversion = 20 msec

Figure 7. CS5 509 S witched- Bridg e Low -power Dig itizer wit h +5 V olt E xcit atio n.

TP0610L

100k

350

Q1, Q2 Siliconix

2N7000

8 AN31REV3

POST

Bridge Transducer Digitizer Circuits

(See Text)

PROCESSOR

SID

SOD

SCLK

DRDY

RST

Serial

Interface

FIR

Channel

Modulator

2-Channel

IN2 OUT2

Delta-Sigma

DGND

0.1

VA- VD-

0.1

-5

Filter

VD+

MDRV-

IN1 OUT1

CS5516

SMODE

CS5520

16 or 20 bits

XOUT

4-bit D/A

4.096 MHz XIN

Calibration

Converter

Gain

Block

1,2,4,8

VA+ MDRV+

0.1 0.1

50 Conversions/sec before averaging

Sync

BX2

Bridge

BX1

VREF-

VREF+

470pF

7.5k

301

7.5k

4.7 nF

25X

AGND1 AGND2

AIN-

AIN+

4.7 nF

301

-5

Figure 8. CS5516/CS5520 Using dc Bridge Excitation.

AN31REV3 9

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