Một phương pháp phân chia analog sử dụng ADC analog cho các mạch điều kiện trạng thái

I, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam Abstract Ivlany measurement circuits use divider of two analog signals, for example temperature measurement circuit using resistive

An Analog Divided Method Using Analog ADC for Conditioning Circuit

Nguyen Thi Lan Huong Hanoi University of Science and Technology, No I, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam

Abstract

Ivlany measurement circuits use divider of two analog signals, for example temperature measurement circuit using resistive bridge; deformation measurement bridge Analog divided circuit is a highly difficult and complex to design This paper presents an analog divided method to obtain digital results by using (SIWAREX) of SIEMENS Results are then compared with tradition method to assert really nght ofmettiod

Key word Divider, ADC, Load cell

1 Introduction

In this paper, a study of advantages and

disadvantages of using ADC as divided circuit The

analyzed in documents or books on the measurement

that has only been menhoned m same of application

the method Results of the experiments show that the

proposed method out-performed traditional one

2 The Method Using Converter ADC as a Divided

Circuit

An ADC output value is calculated hy the

formula

_U^

q

In this equation Ui is an input voltage;

SUPPLY VOLTAGE

(1)

i-'i la ait l u ^ u i v u i m g c ,

q IS a last significant bit

In ADC the q value is derived from the

reference voltage

Thus, the output N of an ADC is a number

proportional to the ratio of the input voltage and the

reference voltage However, the ADC reference

voltage IS interpreted as a constant voltage, both

voltage regulators inside and outside ADC, Hence,

the output value of the ADC is only a varied number

of input voltages

In formula (I) if Um voltage, reference

voltage Vref are considered as two analog variables,

we will have N as a percentage of the ratio of two

analog signals Uin and Vref This divider has result is

numeric This is advantage compare with another

analog divider

' Corresponding aulhor: Tei: (-1-84) 914 949.697

Fig.l The importance signals of ADC Divided circuit two analog signals are widely apphed m the measurement circuit because of its

of load cell is often used for 1-way circuits (for Wheatston - [2]) with one or two actives branch Here we analyze a 1 arm bridge circuit works as follows:

For this scheme an approximate one can calculate

R ^cc

Accordmg to the above formula, we have the resistor raUo is considered as the output voltage of the bridge considered to bridge the supply voltage is constant Therefore result is measurement of diagonal voltage, as it would need to ensure that errors of supplier are small From these observations, there is a more accurate method if we put 2 signals (and the diagonal voltage supply) to the input of the ADC and the standard sources, such as the value of the ADC output IS proportional to the second ratio of input voltage and power supply This model has been used

in [2] without clearly identified its advantages

R4 r / \ y R3

y • \

• " ^ S^ R2

\ y

V

' ' ° ^ R r ^ R 4 ' ' « - R Z

R1 RZ R4 ' R3

AT BALANCE

Vo = 0 IF ^ =

Fig 2 Wheastone Bridge

Experiment and Results Compare

In order to compare two methods, the systems

are implementing based on Load cell SiwaREX of

Siemens

The design of the system is illustrated in the

Fig 3,

Expenments for measuring circuit work in two

modes, the default mode when turn the mode used

ADC normally In this mode, the reference voltage is

the standard voltage, independent of the stability

voltage of the sensor resistance The Second mode of

experiment is ADC as divider

The value received by the ADC

conversion-The conditioning of Load cell illusfrated in

Fig 2,

With bridge measuring circuit the 4 branches

stamped compensation of the sensor as shown in Fig

3 about SIWAREX R BB [3, 4], differential voltage

output value of 2 for the branch:

CT- U^.AR

(3)

UT is the differential output voltage value of

the resistive two branches

UN is power supply In the second mode The

voltage output of coditioning circuit connect lo the

input of ADC, and the power supply connets to

written in C programming language using software

Code Vision for microprocessor ATMEGA8535 This

microprocessor has ADC inside, which 2 mode of

reference voltage The algorithm is illustrated in the

follow Fig 4

Fig 3 Signals of Load cefl Comparisons of experimental results are shown in the following table Survey the independent

of output in changing input values (power supply)

sensor The values of voltage divider obtained to make comparisons Standard equipment Power Supply Kikusui PMC35-3 is used to supply for the the value of the sensor output voltage changes in difference 2 modes

Table 1 Experimental results for testing dependency

of output to power supply in difference in 2 modes

Voltage (V) IslMode (Kg) 2nd Mode (Kg)

4,70

47,45

49.99

4,75

47,98

50,01

4,80

48,28

50,02

4,85

48,74

50,00

4,90

49,15

50,00

4,95

49.57

50.01

5.00

50,01

50,00

From the experiment results in Table I we had foilowmg conclusions;

When there is a slightly change of power supply (0.05 V), there would be a sweeping change in the first working mode While, with the divided mode, the reference voltage for sensor power supply (second mode), the measurement results are almost

no changed

When usmg mode I, to ensure the stability and accuracy of measurement results, the power supply of

Raising the voltage range may be greater than supply stable In fiirther, the supply voltage for die sensor in voltage for ADC ^'"s ""ode is not higher than supply voltage for ADC, When using second mode, the results are accurately

( START j

4 Conclusion

This method is applied to the power supply

circuit with high accuracy The experimental results

of advantage is result of divider is numerical, wich

easy to use in digital systems as in microprocessor or

microcomputer This method only apply for ADC has

outside reference

Using ADC for divided circuit is an efficient

method of conditioning

[I] Data sheet ICL 7107, MAXIM, 2000

[2] Bridge circuits, Walts Kester, Analog Device

Company, 2002

[3] Handbook of modem sensor s (physics Design

and Application), Jacob Fraden, Springer, 2004

[4] SIWAREX weighting system Catalog

SIEMENS 2002