CA3140 Gain Problem With PIC

Suraj143 · Jul 2, 2011

On the below circuit the feedback resister value must be 9K.But with 9K I get those errors.

Op amp output disconnected from PIC ADC
@ 30’C, PT100=111.6 Ohms, op amp output shows 0.515V.

Op amp output connected to the PIC
@ 30’C, PT100=111.6 Ohms, op amp output shows 0.44V.

There is a 4’C difference. So the temperature shows 26’C instead of 30’C.

I changed four CA3140 op amps from different manufactures & all shows different outputs.

The problem solves when I adjust the gain until I get the correct values. But then again when I disconnected the Op amp output from PIC ADC the op amp output value will be too higher.

Why is this happening? How to solve this?

Pommie · Jul 2, 2011

Are you sure you have the pic pin set as input?

Mike.

Suraj143 · Jul 3, 2011

Here how I set up.I use PIC16F886.

Code:

Init_Ports	clrf	STATUS		; B0
		bsf	STATUS,RP0	; B1
		bsf	STATUS,RP1	; B3
		clrf	ANSEL
		clrf	ANSELH
		bcf	STATUS,RP1	; B1
		movlw	b'01100000'
		movwf	OSCCON
		movlw	b'00000001'	; AN0 = Input
		movwf	TRISA
		movlw	b'11110000'
		movwf	TRISB
		movlw	b'00000000'	; Make UART TX as output
		movwf	TRISC
		movlw	b'00001000'	; MCLR
		movwf	TRISE
;---------------------------------------------------------------------------------------------------	
Setup_TMR0	movlw	b'00000011'	; PS=1:16,Global WPU=Enable
		movwf	OPTION_REG
		movlw	b'11110000'
		movwf	WPUB
		bcf	STATUS,RP0	; B0
		movlw	b'00100000'	; T0IE = ON
		movwf	INTCON
		clrf	TMR0		; reset TMR0
;--------------------------------------------------------------------------------------------
Initialise_AD	bsf	STATUS,RP0	; B3
		bsf	STATUS,RP1	; //
		movlw	b'00000001'	; RA0=Analog Input
		movwf	ANSEL		; turn off analog inputs & make it to digital
		clrf	ANSELH
		bcf	STATUS,RP1	; B1
		movlw	b'10000000'	; Right Justified,Vref=Vdd & Vss
		movwf	ADCON1
		bcf	STATUS,RP0	; B0	
		movlw	b'01000001'	; AD=ON,CHS=RA0,FOSC/8
		movwf	ADCON0

There is 70-100mV change in without & with the PIC connected.

KeepItSimpleStupid · Jul 3, 2011

I lost all of the stuff I had lined up. So here is the simple version. One. your not using a ratiometric technique. Your using a voltage regulator rather than a reference. The bridge and the ADC has to share the same reference.

Briefly: https://www.electro-tech-online.com/custompdfs/2011/07/PT100Interface.pdf

It's very important to compensate for distance in a RTD. Usually the measuring systems are 3 or 4 wire.

ericgibbs · Jul 3, 2011

hi Kiss,
If the OP's circuit uses the 5V for the PT100 'bridge' and the PIC uses the same 5V for Vs and has the +Vref internal set as the Vs, the system is ratio metric.

Looking a the program, it shows that the Vs is chosen as the Vref, so its all OK.

CA3140 Gain Problem With PIC

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CA3140 Gain Problem With PIC

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