I need to convert a 4v peak to peak signal (-2v to +2v) into a 0 to 3.3V signal to get it into my ADC on LPC 11u24. I have tried several designs but the problem is that all of the designs I found are based on single rail op amps. I only have dual supply op amps. Any suggestions?
Electronic – Level shift 4v peak to peak to 0 – 3.3V
adcleveloperational-amplifier
Related Solutions
Here are a number of inexpensive op-amps that might work as well. All of them are rail-to-rail output and have an input range including the negative rail. Some of these aren't rail-to-rail in, but with a common-mode range within about 1V of the upper rail, it's fine for an application where you are amplifying the input.
- LMV822 (RRO)
- OPA348 (RRIO)
- OPA336 (RRO, 0.125mV offset, 100kHz GBW which is small but probably fine for thermistors)
- MCP6001 (RRIO)
- MCP601 (RRO, 2mV offset, 2.8MHz GBW)
edit: p.s. your question had inspired me to write an article about thermistor signal conditioning in general; maybe you'll find it helpful.
I think you can do all of this with just two op-amps (although someone will probably correct me).
The first stage is the peak detector and decay. The op-amp is wired to emulate a perfect diode. We take the feedback signal from after the diode so that the op-amp drives hard enough to overcome the voltage drop of the diode. Being a peak detector, it doesn't really eed a rectified signal. The capacitor holds the peak value, while the resistor decays it. You can make the decay slower by choosing a larger capacitor and resistor. And vice versa.
The second stage is simply 15x gain. It has low output impedance.
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Best Answer
Based on this post, a summing amplifier could work. Adjusting the values, the schematic would be as follows:
simulate this circuit – Schematic created using CircuitLab
The output is calculated just like a non-inverting amplifier. Thus, when the sine wave is +2V, the terminals of the op-amp are at 2V. Since
\$V_O = V_I(1+\frac{R_4}{R_3})\$
the output will be 3.3V. When the sine wave is -2V, the terminals will be 0V, since the two voltage sources, \$R_1\$, and \$R_2\$ act as a voltage divider. Then the output will also be 0V.
The caveat here is that you need a rail to rail op-amp, or supply voltages that are less than 0V and greater than 3.3V. Otherwise the op-amp will clip the output.