26
LTC2421/LTC2422
24212f
APPLICATIO S I FOR ATIO
WUUU
nummeas = nummeas + c1%
LOCATE 2, 2: PRINT "Scan#="; nummeas; " "; DATE$; " "; TIME$;
OUT mcr%, c0%: 'Initialize SCLK=0
k1 = km: d2400 = 0: chan% = c0%: signneg% = c0%
FOR bita% = 31 TO 0 STEP -1: v31 = 1
148 GOSUB 2200: v31 = v31 + 1
150 IF bita% = 31 THEN GOTO 152 ELSE 156
152 IF dfrm% = c0% THEN GOTO 156
155 IF v31 > 2 THEN LOCATE 16, 16: OUT port, c0%: PRINT "waiting for eoc":
IF v31 < 20000 THEN IF dfrm% = c1% THEN GOTO 148
IF dfrm% = 1 THEN LOCATE 17, 16: PRINT "Timed out on EOC,not fatal"
FOR bs = 1 TO 32: ' never got an eoc => clock it 32 times
GOSUB 2000: NEXT bs: GOTO 1800
156 LOCATE 16, 16: PRINT" ": GOSUB 2000
IF bita% = 30 THEN 161 ELSE 171 ' CHANNEL BIT !!!!!!!!!!!!!!!
161 IF dfrm% = c1% THEN chan% = c1%: ch1% = c0%
IF dfrm% = c0% THEN chan% = c0%: ch1% = ch1% + c1%
IF ch1% > c4% THEN GOSUB 3700: ch1% = c1%
171 IF bita% = 29 THEN IF dfrm% = c0% THEN signneg% = c1%: ' NEG
IF bita% <= 28 THEN d2400 = d2400 + (dfrm% * k1): k1 = k1 / c2%
NEXT bita%: k1 = 1: digin% = c0%: 'MATH BELOW
1600 PPM = (d2400 / km) * kn: rw% = 6: hz% = (chan% * 20) + 1
IF signneg% = c1% THEN 1700 ELSE 1705
1700 IF d2400 <> c0% THEN PPM = (PPM - 2000000)
1705 LOCATE rw%, hz%: PRINT PPM; " "; : LOCATE rw%, hz% + 11:
PRINT "PPM";
LOCATE rw% + 1, (chan% * 20) + 1: GOSUB 3800: 'THIS WORKS!
1800 LOOP WHILE INKEY$ = "": REM Works with "DO"
GOTO 5000 ’rem END!!-------------- Subs follow !!----------------!!!
1900 ’ESSENTIAL INITIALIZATIONS
REM set some constants, since they can be accessed much faster
LET c128% = 128: c64% = 64: c32% = 32: c16% = 16: c8% = 8: c4% = 4
LET c3% = 3: c2% = 2: c1% = 1: c0% = 0: km = (2 ^ 30) - 1: kn = 1000000
IF cport = 2 THEN OPEN "COM2:300,N,8,1,CD0,CS0,DS0,OP0,RS" FOR
RANDOM AS #1: port = (&H2F8)
IF cport = 1 THEN OPEN "COM1:300,N,8,1,CD0,CS0,DS0,OP0,RS" FOR
RANDOM AS #1: port = (&H3F8)
LOCATE 5, 21: PRINT "CHANNEL 1": LOCATE 5, 2: PRINT "CHANNEL 0"
FOR n% = port TO port + 7: OUT n%, 0: NEXT n%: ’Init UART regs
CLOSE #1: DEF SEG = 0: RETURN ’--------------------------------------
2000 ’SUB read MSR AND RETURN data dfrm% INTERFACE
x3% = INP(msr%) AND c16%: OUT mcr%, c1%
GOSUB 3000: OUT mcr%, c0%
2040 IF x3% = c16% THEN dfrm% = c1% ELSE dfrm% = c0%
OUT mcr%, c0%: RETURN ’---------------------------------------------
2200 ’SUB READ THE DATA BIT dfrm% does NOT change sclock
x3% = INP(msr%) AND C16%: GOTO 2040: RETURN’----------------
3000 REM delay sub !!!!!!!!!!
FOR n8% = 0 TO 1: OUT port, c0%: NEXT n8%: RETURN: ’----------
3700 FOR n = 6 TO 9: LOCATE n, 20
PRINT " ": NEXT n: RETURN’---------------------------
3800 ’SUB to convert PPM into Volts and print it
v = PPM * (5 / 1000000): v1 = v * 1000000: hz% = (chan% * 20) + 12
IF v <= .1 THEN PRINT v1; " "; : LOCATE rw% + 1, hz%: PRINT "uV "
IF v > .1 THEN PRINT v; " "; : LOCATE rw% + 1, hz%: PRINT "Volts";
RETURN’----------------------------------------------------------------
4970 PRINT "ERROR !!!!!!!!!!!!!!!"
5000 PRINT : LOCATE 18, 1: PRINT "Ending!!": PRINT "Hit any key to exit."
PRINT "Start ="; timestart$; " End = "; TIME$; " # samples ="; nummeas
CLOSE #1: END
Single Ended Half-Bridge Digitizer
with Reference and Ground Sensing
Sensors convert real world phenomena (temperature, pres-
sure, gas levels, etc.) into a voltage. Typically, this voltage
is generated by passing an excitation current through the
sensor. The wires connecting the sensor to the ADC form
parasitic resistors R
P1
and R
P2
. The excitation current also
flows through parasitic resistors R
P1
and R
P2
, as shown in
Figure 29. The voltage drop across these parasitic resis-
tors leads to systematic offset and full-scale errors.
In order to eliminate the errors associated with these para-
sitic resistors, the LTC2421/LTC2422 include a full-scale
set input (FS
SET
) and a zero-scale set input
(ZS
SET
). As shown in Figure 30, the FS
SET
pin acts as a
zero current full-scale sense input. Errors due to parasitic
