Building a Battery Operated Auto Ranging DVM with the ICL7106 Application Note

AN046 Larry Goff

Introduction

Input Divider Network

In the field of DVM design, three areas are being addressed with vigor: size, power dissipation, and novelty. The handheld portable multimeter has gained in popularity since low power dissipation devices enabled battery operation, LSI A/D converters reduced IC count, and novelties such as conductance, automatic range scaling, and calculating were included to entice the user.

A simplified drawing of the divider network is shown in Figure 2. This configuration was chosen for simplicity and implementation using analog switches. The low leakage ID101s are used for input protection, and the second set of switches to IN LO reduces the net error due to switch resistance. This can be seen calculating IN HI and IN LO voltages for the two equivalent circuits.

This application note describes a technique for auto-ranging a battery operated DVM suitable for panel meter applications. Also, circuit ideas will be presented for conductance and resistance measurement, 9V battery and 5V supply operations, and current measurement.

For equivalent circuit A,

Auto Ranging Circuitry The control signals necessary for auto-ranging are overrange, under-range, and clock. The over-range and underrange inputs control the direction of a scale shift, becoming active at the completion of an invalid conversion and remaining active until a valid conversion occurs. The clock input controls the timing of a scale shift. This signal should occur only once per conversion cycle, during a time window which will not upset an ongoing conversion and must be disabled after valid conversions. In the circuit of Figure 1, inverted over-range (O/R) and under-range (U/R) are generated by detecting the display reading. The ICL7106 turns the most significant digit on and blanks the rest to indicate an over-range. An under-range occurs if the display reads less than 0100. R1C1 and R2C2 are required to deglitch O/R and U/R. The next step in the logic disables O/R and U/R prior to shifting into nonexistent ranges. O/R is disabled when in the 200V range, while U/R is disabled when in the 200mV range. The next level of gating disables the clock if the conditions are as described above and a valid conversion state exists. Clock is enabled only when a range shift is called for and there exists a valid range to shift into. The CD4029 is a four bit up/down counter, used as a register to hold the present state and as a counter to shift the scale as directed by the control inputs. The CD4028 is a BCD to decimal decoder interfacing the CD4029 and ladder switches. An additional exclusive OR gate package is added to drive the appropriate decimal point.

1

 RS + R ⁄ K  V MEAS = V IN HI =  ------------------------------------- V IN  R S + R + R ⁄ K

(EQ. 1)

where RS = switch resistance, R = input resistance (1MΩ), and 1 + K is the desired divider ratio. Ideally VINHI should be R⁄K 1 V IDEAL =  ----------------------- V IN =  ------------- V IN  R ⁄ K + R  1 + K

(EQ. 2)

Therefore the percent error is: Ideal – Actual --------------------------------------- 100, Ideal

(EQ. 3)

R S + R/K   or  1 – ( 1 + K ) ----------------------------------- 100 R  S + R/K + R

(EQ. 4)

The worst case error occurs at (1+K) = 1000. For this example, the error due to a 1kW switch resistance is 99.7%. IN HI for equivalent circuit B is the same as Equation 1. However, IN LO for circuit B is: RS    ------------------------------------- V IN ,  R S + R + R ⁄ K

(EQ. 5)

and combining Equations (1) and (5) R⁄K V MEAS = V INHI – V INLO =  ------------------------------------- V IN  R + R + R ⁄ K S

(EQ. 6)

The percent error is equal to: R/K  1 – ( 1 + K ) ---------------------------------- 100  R + R + R/K

(EQ. 7)

S

Using the same values for RS, (1+K), and R, the worst case error is 0.1%. This error can be further improved if lower rDS(ON) switches are used. From the results calculated above, the worst case conversion error due to switch resistance will be one count of the least significant digit for a full scale input, and a slight adjustment to R itself will correct the remaining error on all scales.

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© Intersil Corporation 1999

V+

V+

1

V+

OSC 1

40

2

D1

OSC 2

39

3

C1

OSC 3

38

4

B1

TEST

5

A1

REF HI

36

6

F1

REF LO

35

7

G1

CREF

34

8

E1

CREF

33

9

D2

COMMON

32

10 C2

IN HI

31

CREF 100kΩ 100pF CLOCK

37 DIG GND

A/Z

1kΩ 1µF

2

ICL7106 PIN26 V-

1 8 9 74C32

10 4011

4011 3 13

5

1 2

6

4 A

12

O/RANGE 3

2

10kΩ 6 R1

C1

4023 5 10 0.005µF TEST O/RANGE 1 10kΩ 2

R2 9 4023 C2 0.005µF

1 4023

11 4 12

5 6

13

8 9

10

8

IN LO

30

12 A2

A-Z

29

13 F2

BUFF

28

14 E2

INT

27

15 D3

V-

26

16 B3

G2

25

17 F3

C3

24

18 E3

A3

23

19 AB4

G3

22

20 POL

BP

21

Q2 D

R8

22kΩ

Q1

0.47µF

100kΩ

S

47kΩ

3N169 N CH.

1MΩ

D2

0.22µF V-

TEST

11

13

VIN

4011

11

2N3702 Q3

D

12

TEST

4011

2

3 4

11 B2

0.01µF

S

D1

47MΩ

C3 0.1µF

20kΩ

5.1kΩ

C ID101 TEST

V+ 1

PE

V+ 16 CLK 15

2

14

3 4

V+

2 3

13

4

4 B A

CD402T

5

C1

12

5

C 12

6

Q1

Q7 11

6

D 11

UP/DOWN 10

7

A 10

BINARY 9 DECODE

8

6, 12

TEST

1

A 7 8

V

8 CD4016

B 13

CD4029BC

1

2

B

C

5

6 8

2

9 12

13

5, 13 12

CD4016

TEST UP/DOWN COUNTER

BACK PLANE DECODER

ARROW

FIGURE 1. AUTO RANGING CIRCUITRY

R4

9

10.1kΩ 10 11 R5 3 4

A OR D

9

V

1MΩ

VIN

1 14

O

1.001kΩ 3

13, 5

3 15

2

R6

D

V+ 16

1

R1 R2 R3

8 6

111.1kΩ

10 11 9

OPEN OPEN

Application Note 046

D 3

24kΩ

Application Note 046

R IN HI

+ ID101 VIN

R/999

R/99.01

R/9

IN LO

SWITCH CONTROL LINES 200V

20V

200mV

2V

FIGURE 2A.

+

+ R

R

TO IN HI

TO IN HI VIN

VIN

R/K

R/K

VMEAS TO IN LO

VMEAS R SWITCH

-

R SWITCH TO IN LO

FIGURE 2B. EQUIVALENT CIRCUIT A (SWITCHES TO IN LO REMOVED)

-

FIGURE 2C. EQUIVALENT CIRCUIT B (SWITCHES TO IN LO INCLUDED) FIGURE 2. INPUT DIVIDER NETWORK

Ranging Clock Circuit Two N-Channel MOSFETs, a PNP transistor and a handful of passive components combine to generate the clock signal used to gate the auto-ranging logic. A closer look at the inner workings of the ICL7106 will help clarify the discussion of this circuit. The analog section of the ICL7106 is shown in Figure 3. It can be shown that CREF low (pin 33 of ICL7106) will sit at -VREF for DE+ and at common for DE-, with DE+ designating the deintegrate phase for a positive input signal and DE- referring to a negative input signal. During the autozero phase, CREF low is tied to an external reference through pin 35, which in Figure 1 is VREF below the positive supply. The net result is that CREF low is above COMMON during auto-zero, is left to float during signal integrate, and is at or below COMMON during deintegrate. R8 and D1 are added externally to pull CREF to COMMON during integrate, with Q2 and R1 included to speed this action. The signal at CREF low is now a square wave that is high during auto-zero and low at all other times. Q1 and Q3 amplify and level shift this waveform for logic level compatibility. This clock signal is

3

gated through D2 and controls the timing of the auto-ranging circuitry. C3 is added to delay the clock, eliminating disparity with O/R and U/R (see Figure 4 for timing diagram).

Application Note 046

CREF RINT CREF+

REF HI

34

36

V+

A-Z

REF LO

CREF -

BUFFER

35

33

28

A-Z

CAZ V+ 1

CINT

A-Z

INT

29

27 INTEGRATOR

-

+

10µA

-

+

TO DIGITAL SECTION

-

+

2.8V

31 IN HI DE-

INT

DE+

6.2V

INPUT HIGH

A-Z

A-Z

DE+

32

COMPARATOR

-

N

+

DE-

COMMON INT

INPUT LOW

A-Z AND DE(±)

30 IN LO

26 V-

FIGURE 3. ANALOG SECTION OF ICL7106

INTEGRATOR A/Z

OVER-RANGE CONVERSION

A/Z

VALID CONVERSION

A/Z

CREF LOW (PIN 33)

CLOCK

O/R, U/R

FIGURE 4. TIMING DIAGRAM

Supply Requirements The circuit of Figure 1 operates on a standard 9V transistor battery. CMOS logic and a CMOS A/D converter (ICL7106) are used to extend battery life; the approximate power drain for this circuit is 8mW. The circuit in Figure 5 can also be added to detect low supply voltage. The circuit of Figure 6 can be used to generate ±5V from a single 5V supply. The ICL7660 is a voltage converter which takes a 5V input and produces a -5V output. With respect to common mode signals, the circuit of Figure 1 will have infinite common mode handling capability if operated from a floating 9V battery. However, if powered by a fixed supply such as in Figure 6, the common mode capability of the

4

converter will be limited to approximately ±2V, if COMMON is disconnected from -VIN.

Application Note 046 For transconductance measurement, merely switch RSTD and RX. This scheme makes the measurement of large resistors, in conductance form, convenient and easy. This is also convenient for leakage measurements.

1MΩ 1

8 ICL8211

2

7

3

6

4 FIG 7

5

1MΩ

180K TO DISPLAY INDICATOR

A simple current meter can be built using the circuit of Figure 8. The low leakage of the ICL7106 (10pA/max) makes possible the measurement of currents in the mid pico-Amp range. However, the switch leakage current will limit the accuracy of the resistor network and may degrade converter resolution.

BACKPLANE TEST

FIGURE 5. LOW VOLTAGE DETECTOR 900R

+5V NC 1

8

2

AUTO-RANGING DVM CIRCUIT

ICL7660

+ 10µF

IN HI 90R

7 NC 6 NC

3

-

IIN

ICL7106 9R

4

5

IN LO 10µF

-5V R

FIGURE 6. GENERATING ±5V FROM +5V

Resistance, Transconductance and Current Circuits

FIGURE 8. CURRENT METER

The purpose of this section is to show the simplicity of measuring transconductance (1/R) and resistance with the ICL7106. The circuit of Figure 7 requires only one precision resistor per decade range of interest. The conversion output is described by the formula:  RX   --------------- 1000  R STD

(EQ. 8)

V+

REF HI RSTD REF LO IN914 or IN4148

X4

ICL7106 IN HI

RX IN LO

COMMON

FIGURE 7. TRANSCONDUCTANCE AND RESISTANCE MEASUREMENT

5

Using the ICL7126 and ICL7107 With a few modifications, the circuit of Figure 1 can easily be adapted for use with either the low power ICL7126 or the ICL7107. Using the ICL7126 simply requires a change in the values of the integrating and auto-zero components. Refer to the ICL7126 data sheet for details. The ICL7107 is an LED version of the ICL7106, and is a bit trickier to use in this application. First the over-range/underrange logic must be changed slightly. Simply replace the quad exclusive-NOR with an LM339; connect the outputs, as before, to the CD4023 triple 3-input NAND. Second, the ICL7107 requires +5V and -5V rather than the +9V battery used in Figure 1. If battery operation is desired, the negative supply can be derived from 4 Ni-Cad cells in series and an ICL7660 (see Figure 9). Note that both supplies float with respect to the input terminals. (Logic supplies are V+ and DIG. GND.)

Application Note 046 ICL7107 +5V ICL7660 1

8

2 + 10µF

-

7

3

6

4

5

-5V +

10µF

12kΩ

+ -

O /RANGE

+ +

U /RANGE

CD4023 OR 74C10

1 V3

OSC 1 40

2 D1

OSC 2 39

3 C1

OSC 3 38

4 B1

TEST 37

5 A1

REF HI 36

6 F1

REF LO 35

7 G1

CREF 34

8 E1

CREF 33

9 D2

COMMON 32

10 C2

IN HI 31

11 B2

IN LO 30

12 A2

A-Z 29

13 F2

BUFF 28

14 E2

INT 27

15 D3

V- 26

16 B3

G2 25

17 F3

C3 24

18 E3

A3 23

19 AB4

G3 22

20 POL

GND 21

+ LM339

-5V

NEGATIVE (0V) LOGIC SUPPLY

33kΩ

FIGURE 9. CIRCUIT FOR DEVELOPING UNDERRANGE AND OVERRANGE SIGNALS FROM ICL7107 OUTPUTS. THE LM339 IS REQUIRED TO ENSURE LOGIC COMPATIBILITY WITH HEAVY DISPLAY LOADING

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