E007S34.PDF

COM PUTER

I/U curve tracer

driven via the PC printer port

Source: Maxim Integrated Products

With a set of Maxim A/D and D/A converters and a handful of other components,

you can make a curve tracer that can be driven from the printer port of a PC. A

simple BASIC program takes care of the communication between the PC and the

measurement circuit, and also converts the measured values into graphic form for

display on the monitor.

Not long ago, it was common practice to mea-

sure the I/U characteristics of a semiconduc-

tor device using an X-Y oscilloscope and a

suitable measuring circuit. An electronics

engineer can extract a lot of information

about the operation of the semiconductor

from these curves.

In the present computer age, the I/U curve

may well have dropped into obscurity, but it

is still useful for evaluating a number of spe-

ciﬁc semiconductor characteristics. The only

difference is that we no longer need an oscil-

loscope, since we can instead employ the ver-

satile PC.

With the measurement circuit presented here,

which comes from Maxim, the I/U character-

istics of a semiconductor or IC can be mea-

sured and then displayed on the monitor. This

is made possible by the use of two serial-

interface ICs: a 12-bit DAC and a 12-bit ADC.

A short BASIC program looks after controlling

the circuit and displaying the data on the

screen. Since the circuit is designed for use

with a standard PC printer port, the interac-

tion between the circuit and the computer is

very simple.

test current for the device being

measured.

The current ranges from ±40 µA to

±40 mA, depending on the value

selected for the resistance of R SENSE

(100

, 1 k

Ω

, 10 k

Ω

or 100 k

Ω

). The

maximum output current is approxi-

mately equal to the value of the out-

put voltage of IC6a (4.096 V) divided

by the value of R SENSE .

The current through the DUT (device

under test) produces a voltage drop

15V

± 4V096

CLR

22k

± 2V048

IC7

HA-5221

DOUT

IC4

RFB

OUT

DIN

SCLK

IC6a

MAX531

REFIN

REFOUT

BIPOFF

PROBE

DUT

15V

AGND DGND

to PC

33µ

FORCE

SENSE

11k

R SENSE

SHDN

3k3

IC6b

DOUT

22k

SCLK

IC3

MAX189

REF

GND

22k

15V

IC5a

IC5b

IC1

15V

SENSE

± 2V048

The hardware

OUT2

MAX663

OUT1

± 4V096

SHDN

GND

SET

15V

Figure 1 shows the schematic diagram of the

hardware. IC4 is a 12-bit D/A converter that

is conﬁgured for a bipolar output voltage with

a range of plus and minus 2.048 V. The opamp

IC6a, which is connected to the V out pin,

ampliﬁes this voltage by a factor of 2, so that

a voltage ranging between plus and minus

4.096 V is present at the output of IC6a. IC7

in turn converts this voltage into a current

that is proportional to the voltage. This is the

22µ

IC2

SHDN

IC5 = MAX492

IC6 = MAX478

GND

SET

22µ

IC5

IC6

PLI

SHDN

OUT1

MAX664

SENSE

OUT2

15V

000028 - 11

Figure 1. The main components of the circuit are a set of Maxim serial-interface

A/D and D/A converters.

Elektor Electronics

7-8/2000

Ω

COM PUTER

‘LISTING 1 - BASIC PROGRAM

‘LPT1 OUT @ &H378, IN @ &H379

‘LPT2 OUT @ &H278, IN @ &H279

‘25 WAY D TYPE

‘SIGNALS TO CIRCUIT

‘PIN 4 D2 (OUT) SCLK

‘PIN 5 D3 (OUT) DATA OUT (FROM PC)

‘PIN 6 D4 (OUT) CS\

-1V

F1 TERMINATES

-2V

-3V

-4V

-I MAX

0mA

+I MAX

‘SIGNALS FROM CIRCUIT

‘PIN 15 D3 (IN) DATA IN (TO PC). ALSO SIGNALS EOC

‘PINS 18-25 ARE GROUND

000028 - 12

‘INTRODUCTION

PRINT ”WELCOME TO THE MAXIM CURRENT-VOLTAGE CURVE TRACER”

INTRO:

PRINT ”WHERE IS THE CIRCUIT CONNECTED? ENTER 1 FOR LPT1 OR 2 FOR LPT2”

INPUT P%

IF P% = 1 THEN PORT& = &H378 ‘SET LPT1

IF P% = 2 THEN PORT& = &H278 ‘SET LPT2

IF P% = 1 OR P% = 2 THEN GOTO INTRO1 ‘TRAP ERROR

PRINT ”WRONG!!!!!! PLEASE TRY AGAIN”

GOTO INTRO

INTRO1:

-1V

F1 TERMINATES

-2V

-3V

-4V

-I MAX

0mA

+I MAX

000028 - 13

Figure 2. Two examples of measured

I/U curves: (a) shows a Schottky

diode, while (b) shows a more

complex analogue IC.

ON KEY(1) GOSUB FINISH ‘F1 EXITS

KEY(1) ON

MAIN: ‘MAIN BIT OF PROGRAM

‘DEFINE SOME VARIABLES

DIM Y(512) ‘ARRAY TO HOLD PLOT DATA

DIM DIN(12) AS INTEGER, DOUT(12) AS INTEGER ‘DATA IN AND DATA OUT

across the component. This voltage

is measured by the difference ampli-

fier IC6b. In order to avoid an offset

error that depends on the value of the

selected current-sensing resistor, the

signal for the inverting input of this

opamp is taken from the low-imped-

ance non-inverting input of IC7. The

disadvantage of this is a fixed error

equal to the input offset of IC7.

The amplification of the difference

amplifier, in combination with the

added offset, yields a maximum out-

put swing of 0 to 4.096 V. This is

exactly the right range for the unipo-

lar input of the 12-bit A/D converter

(IC3). The 3.3-kΩ resistor in series

with the input of the ADC limits the

input current in case the output volt-

age of IC6b should be greater than

the supply voltage of IC3.

When a measurement is being

made, the software drives the DAC

such that it produces a ‘staircase’

current. The resulting voltage across

the DUT is measured by the ADC

and then displayed on the PC moni-

tor, with a resolution of 640 x 480 pix-

els. The two examples shown in Fig-

ure 2 illustrate some typical results.

A resolution of 12 bits is actually

somewhat luxurious for this applica-

‘SET UP DISPLAY FOR OUTPUT

CLS 0

SCREEN 12 ‘VGA SCREEN

WINDOW (-120, -55)-(520, 435) ‘DEFINE WINDOW CO-ORDINATES

LINE (0, -5)-(0, 410), 1 ‘DRAW A FEW LINES

LINE (-5, 0)-(512, 0), 1

LINE (512, -5)-(512, 410), 1

LINE (-5, 410)-(512, 410), 1

LINE (-5, 205)-(512, 205), 1 ‘CENTRAL AXES

LINE (256, -5)-(256, 410), 1

LINE (-5, 51)-(0, 51), 1 ‘MARKERS

LINE (-5, 102)-(0, 102), 1

LINE (-5, 153)-(0, 153), 1

LINE (-5, 256)-(0, 256), 1

LINE (-5, 308)-(0, 308), 1

LINE (-5, 359)-(0, 359), 1

LINE (64, -5)-(64, 0), 1

LINE (128, -5)-(128, 0), 1

LINE (192, -5)-(192, 0), 1

LINE (320, -5)-(320, 0), 1

LINE (384, -5)-(384, 0), 1

LINE (448, -5)-(448, 0), 1

COLOR 9 ‘LABELS

LOCATE 1, 1, 0: PRINT ”MAXIM CURRENT-VOLTAGE CURVE TRACER”

LOCATE 20, 1, 0: PRINT ”F1 TERMINATES”

LOCATE 2, 12, 0: PRINT ”+4V”

LOCATE 15, 13, 0: PRINT ”0V”

LOCATE 27, 12, 0: PRINT ”-4V”

LOCATE 28, 15, 0: PRINT ”-Imax”

LOCATE 28, 46, 0: PRINT ”0mA”

LOCATE 28, 75, 0: PRINT ”+Imax”

LOCATE 10, 1, 0

PRINT ” Rs Imax”

PRINT ”100R 40mA”

PRINT ” 1K 4mA”

PRINT ” 10K 400uA”

PRINT ”100K 40uA”

START: ‘START OF PLOT ROUTINE

IDATA& = &H10 ‘INITIALISE PORT, SCLK=0, CS\=1

OUT PORT&, IDATA&

Z& = 8 ‘INITIALISE MAX531

7-8/2000

Elektor Electronics

COM PUTER

GOSUB IO

tion, but the software can be modi-

ﬁed to work with higher resolutions,

and you could even add a loop to the

software for this.

Now we come to a few practical

remarks regarding the power supply.

IC7 and IC6 need a symmetrical sup-

ply voltage of ±15 V in order to oper-

ate over the desired range. All other

ICs need only a ±5 V supply. The

voltage regulators MAX663 and

MAX664 (IC1 and IC2) provide the

necessary voltages.

The QBASIC program shown in the

accompanying listing starts by ask-

ing which printer port the circuit is

connected to (1 or 2). After this, the

graphic plot of the I/U curve of the

device being tested appears on the

screen. You can exit the program by

pressing the function key F1. Make

sure to run the program in true DOS

mode, rather than in a DOS box

under Windows. QBASIC can usually

be found in a folder labelled

‘OLDMSDOS’ on the Windows CD, or

else you should use an old version of

MS-DOS.

FOR X& = 1 TO 511 STEP 1 ‘X& IS X AXIS FOR PLOT

Z& = 8 * (X& + 1) ‘NEXT VALUE FOR MAX531

GOSUB IO

PSET (X&, Y(X&)), 0 ‘RESET BACKGROUND

IF Y(X&) = 0 THEN PSET (X&, Y(X&)), 1 ‘CATCH BOTTOM LINE AT START

IF Y(X&) = 205 THEN PSET (X&, Y(X&)), 1 ‘CATCH HORIZ CENTRE LINE

IF X& = 256 THEN PSET (X&, Y(X&)), 1 ‘CATCH VERT CENTRE LINE

Y(X&) = INT(ODATA& / 10)

IF Y(X&) > 408 THEN Y(X&) = 409 ‘CHECK FOR OVER-RANGES

IF Y(X&) < 1 THEN Y(X&) = 1

PSET (X&, Y(X&)), 4 ‘SET NEW VALUE

NEXT X&

GOTO START ‘AROUND AGAIN

IO: ‘DATA I/O ROUTINE

ODATA& = &H0

IDATA& = IDATA& AND &HEF ‘CS\ SET TO 0

OUT PORT&, IDATA&

WAIT (PORT& + &H1), &H8 ‘WAIT FOR EOC FROM CIRCUIT

‘SCLK CYCLE TO SHIFT 189 DATA TO DOUT

IDATA& = IDATA& OR &H4 ‘SCLK TO 1

OUT PORT&, IDATA&

IDATA& = IDATA& AND &HF3 ‘SCLK TO 0

OUT PORT&, IDATA&

FOR SHIFT = 11 TO 0 STEP -1

DOUT(SHIFT) = (INP(PORT& + &H1) AND &H8) ‘GET DATA FROM MAX189

ODATA& = ODATA& + DOUT(SHIFT) * 2 ^ (SHIFT - 3)’COMPILE DATA

IDATA& = IDATA& OR ((Z& AND (2 ^ SHIFT)) / (2 ^ SHIFT) * 8)

‘DATA TO GO TO MAX531

OUT PORT&, IDATA&

IDATA& = IDATA& OR &H4 ‘SCLK TO 1

OUT PORT&, IDATA&

IDATA& = IDATA& AND &HF3 ‘DATA BIT BACK TO 0, SCLK TO

OUT PORT&, IDATA&

NEXT SHIFT

IDATA& = IDATA& OR &H10 ‘CS\ SET TO 1

OUT PORT&, IDATA&

(000028-1)

The BASIC program ‘I-V Curve Tracer’ was writ-

ten by Terrry Millward of Maxim UK, and is avail-

able at the Maxim web site:

www.maxim-ic.com/TechSupport/other.htm

RETURN

FINISH:

END

Serial-interface converters

The simplicity of the circuit described in this arti-

cle is primarily due to the use of a set of serial-

interface converters, namely the MAX 189 and

MAX 1531. The MAX 189 is a serial A/D con-

verter with a resolution of 12 bits that works

with a single +5 V supply. Its input voltage range

also lies between 0 and +5 V. The core of this IC

is a successive-approximation ADC with a con-

version time of 8.5 µs. This is complemented by

a fast sample and hold circuit (1.5 µs), an on-chip

clock generator and a fast serial three-wire inter-

face (see the block diagram in Figure A ).

The conversion rate of the MAX 189 is 25 kilo-

samples/s. Thanks to its built-in interface, this IC

can be easily connected to a PC or microprocessor, as can be seen in this application. In contrast to its companion MAX 187, the MAX 189

does not have an internal reference, but this is not necessary in this application since the DAC that is used provides the reference voltage.

The very low operating power consumption (7.5 mW) makes this IC very suitable for battery-powered applications. In the shutdown mode,

the power consumption drops to only 10 µW.

The MAX 189 is available in an 8-pin DIP package and a 16-pin SOI package.

The MAX 1531 can be seen as a complement to the MAX 189. Its block diagram is shown in Figure B . This 12-bit D/A converter also

works with a single +5 V supply. The MAX 1531 was speciﬁcally chosen for this application, which requires a bipolar output voltage, since

it can also work with a symmetrical ±5 V supply. The current consumption of the MAX 1531 is only 260 µA, including the internal

2.048 V reference source. The IC is housed in a 16-pin DIP or SOI package. The offset voltage, ampliﬁcation and linearity are adjusted

during manufacturing, so the user does not have to be concerned with them.

The internal output opamp of the MAX 1531 can be conﬁgured for an ampliﬁcation factor of 1 or 2, and for a unipolar or bipolar output

voltage. An internal shift register stores the serial data supplied to the IC.

Elektor Electronics

7-8/2000

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