Appendix 2: Signal Conditioning Circuit - with Op-Amp

Water Resources - Experiment #1 Appendix 2: Signal Conditioning Circuit - with op-amp The glass electrode is but one component of our pH sensing system. Sensor systems generally include, in addition to a transducer (the glass electrode), a signal conditioning component, a data transmission component and, back home, a data readout/storage subsystem. We are concerned here with our signal conditioning subsystem. Because the output impedance of the glass electrode is so high, perhaps on the order of 1010 ohms, the device, considered as a voltage source, would be “loaded down” by any ordinary readout device presenting lower impedance - e.g., our digital voltmeter - and hence the voltage the latter displayed would not be a true measure of pH. To ensure we do get a true reading, we use an operational amplifier (“op-amp”) that presents a high impedance, much higher than that even of the electrode, to the electrode’s output. In this case, the op-amp will draw very little current - in fact we assume it draws none - so the input to the op amp will be the true voltage. The circuit is designed such that the output of the op-amp is equal to its input and delivers adequate current to our digital voltmeter, readout. How this works we now attempt to explain. A schematic of the signal conditioning circuit is shown feedback loop in the figure. To understand its function we note first that an op-amp, in open loop mode (no feedback loop) has +4.5v enormous gain - on the order of 105 or 106: That is, if the inverting inp. Vout non-inverting inp. voltage difference between the noninverting and invert Velectrode ing inputs differs from zero - say the noninverting volt age input exceeds the voltage at the inverting input then - 4.5v Vout will grow positive - big time1. (Actually it will grow “split” supply voltage = +/- 4.5v until it reaches the supply voltage of +4.5 volts and be nailed there). Conversely, if the noninverting voltage input drops below the voltage at the inverting input - Vout will grow negative, big time. Now consider what happens with the feedback loop in place - connecting the output to the invert ing input - so that Vout is held to the voltage at the inverting input. (The resistance of the feedback line is taken as zero). If the voltage at the noninverting input differs from the voltage at the invert ing input - say it increases relative to the latter - then Vout will increase due to the op-amp’s gain. But it can’t increase “big time” because once it, and consequently the inverting input voltage, reaches the voltage at the noninverting input, growth must cease for any further increase would produce a noninverting voltage dropping below the voltage at the inverting input and Vout would 1. The labeling of the two inputs to the op-amp as “inverting” and “noninverting” will be explained in time. Roughly speaking, the output of the op-amp is out of phase with an inverting input by 180o while the noninverting input is in phase with the output. The plus and minus signs do not mean that a negative voltage must be supplied to the inverting input, nor a positive voltage to the non-inverting input. 1 correspondingly decrease. The result is that for changing Velectrode the output voltage “follows” the input voltage1. Vout = Velectrode Since the op-amp presents such a high impedance to the glass electrode (in fact the argument above assumed infinite impedance) the Velectrode is truly Velectrode and we have successfully condi tioned our raw signal as presented by our transducer. 1. An op-amp used in this way is called a “follower”. Ref: P. Horowitz, W. Hill, The Art of Electronics, Cambridge University Press, 1980, p. 176 (1999 edition). 2 Circuit board layout. R1 +10v Rg4 + V. R2 c G4 G3 a 2 Rg3 R3 e1 b 3 Rg2 G1 4 1 5 output 6 R4 G2 d 7 -V Rg1 e2 H1 U1 H2 out + gnd _ 1 2 3 4 R POT 1 POT 2 e1 e2 a a b b d c c d 1 4 3 2 d a G1 a c G4 RG c b G3 b d G2 3 Top figure is circuit diagram as it would appear in a text book. The bottom figure is the printed cir cuit board layout. (Overwritten to make readable). . R1 Rg4 + V. R2 G4 c G3 a b Glass Electrode Input (G2) G2 G1 - Rg3 e1 R3 2 3 7 4 1 5 output 6 + Rg2 d R4 -V Rg1 e2 The above shows the signal conditioning circuit for use with the Glass Electrode. The full circuit, including the “bridge” will be used in the structure’s lab. 4