Hi!
I'm trying to design an absolute value circuit with single supplied opamps. I have read the SBOA068 Application note from TI (available online) and tried to simulate the circuit attached below in LTSpice. This circuit is proposed in the app note from TI in figure 5. Unfortunately, by observing the output waveform (where V(n002) is the output voltage) I presume that some error exists, either in the schematic or my design. As you can see, the output is zero when Vin is positive and an inverted version of Vin, when Vin is negative.
Reading the analysis at the SBOA068 app note it says the following :
1) When Vin < 0, then D1 is forward biased and holds the non-inverting input of U2 at virtual ground. Thus U2 operates as an inverting unity gain amplifier.
(So far, so good. My output follows exactly this description.)
2) When Vin > 0, then D1 is reverse biased and thus has no influence on the circuit. And U2, R1, R2, R3 operate as a voltage follower.
Well, this is not true to me. If D1 is removed from the circuit, I cannot see any voltage follower. The non-inverting input of U2 is again tied to ground and U2 operates again as inverting amplifier. The difference is that the output cannot go negative (due to the single supply operation) and thus the output is zero.
Does anyone know where the error in the schematic is? Or maybe in my thoughts above.
I'm trying to design an absolute value circuit with single supplied opamps. I have read the SBOA068 Application note from TI (available online) and tried to simulate the circuit attached below in LTSpice. This circuit is proposed in the app note from TI in figure 5. Unfortunately, by observing the output waveform (where V(n002) is the output voltage) I presume that some error exists, either in the schematic or my design. As you can see, the output is zero when Vin is positive and an inverted version of Vin, when Vin is negative.
Reading the analysis at the SBOA068 app note it says the following :
1) When Vin < 0, then D1 is forward biased and holds the non-inverting input of U2 at virtual ground. Thus U2 operates as an inverting unity gain amplifier.
(So far, so good. My output follows exactly this description.)
2) When Vin > 0, then D1 is reverse biased and thus has no influence on the circuit. And U2, R1, R2, R3 operate as a voltage follower.
Well, this is not true to me. If D1 is removed from the circuit, I cannot see any voltage follower. The non-inverting input of U2 is again tied to ground and U2 operates again as inverting amplifier. The difference is that the output cannot go negative (due to the single supply operation) and thus the output is zero.
Does anyone know where the error in the schematic is? Or maybe in my thoughts above.
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