Hello everyone,
My current project with the T3.6 is a basic oscilloscope, and because the ADC's are only capable of 0V-3.3V measurement, I wondered how I can step down higher (and lower) voltages like ±12. My first thought was using op-amps even though I don't understand them fully yet, but playing with them in an online simulator gave me this circuit:
The first op amp is stepping down the voltage in the inveting configuration, according to R5 which can be a 10k digital potentiometer controlled by the teensy.
for example:
5V-->3.3V
A(gain) = Vout/Vin
A = -3.3/5 = -0.66
A = -Rf/Rin
Rin = 10000
-0.66 = -Rf/10000
Rf = 6600
R5 = 6600Ω.
The second op amp inverts the signal back and (optionally) applies a 1.65V (3.3/2) dc offset so AC can get measured too. This makes the 0V point of the input to appear as 1.65V at the output. (not sure why the voltage at the non-inverting input should be 0.825V (3.3/4) and not 1.65V for this offset to occure, but it seam to work this way).
All of the control over this can be implemented in software, but I have two questions yet:
1. Is this circuit bandwidth-friendly? The sampling rate I'm getting out of the teensy is ~800msps, which means I would like this circuit to "tolerate" around 1mhz bandwidth (at least).
I am not sure what are the calculations for the max bandwidth in op amps, so I will be glad if you help me here!
2. What input protection should I add to the ADC input? for example if R5 is set for 12V-->3.3V but a 24V signal is accidentaly apllied, the ADC input voltage will be 6.6V!
I've read about different types of input protections, but not sure which is the best for this scenario, since bandwidth is critical and I don't want to use a method that will block lower-than-1mhz frequencies.
Hope I was clear enough (tell me if not),
128ITSH
My current project with the T3.6 is a basic oscilloscope, and because the ADC's are only capable of 0V-3.3V measurement, I wondered how I can step down higher (and lower) voltages like ±12. My first thought was using op-amps even though I don't understand them fully yet, but playing with them in an online simulator gave me this circuit:
The first op amp is stepping down the voltage in the inveting configuration, according to R5 which can be a 10k digital potentiometer controlled by the teensy.
for example:
5V-->3.3V
A(gain) = Vout/Vin
A = -3.3/5 = -0.66
A = -Rf/Rin
Rin = 10000
-0.66 = -Rf/10000
Rf = 6600
R5 = 6600Ω.
The second op amp inverts the signal back and (optionally) applies a 1.65V (3.3/2) dc offset so AC can get measured too. This makes the 0V point of the input to appear as 1.65V at the output. (not sure why the voltage at the non-inverting input should be 0.825V (3.3/4) and not 1.65V for this offset to occure, but it seam to work this way).
All of the control over this can be implemented in software, but I have two questions yet:
1. Is this circuit bandwidth-friendly? The sampling rate I'm getting out of the teensy is ~800msps, which means I would like this circuit to "tolerate" around 1mhz bandwidth (at least).
I am not sure what are the calculations for the max bandwidth in op amps, so I will be glad if you help me here!
2. What input protection should I add to the ADC input? for example if R5 is set for 12V-->3.3V but a 24V signal is accidentaly apllied, the ADC input voltage will be 6.6V!
I've read about different types of input protections, but not sure which is the best for this scenario, since bandwidth is critical and I don't want to use a method that will block lower-than-1mhz frequencies.
Hope I was clear enough (tell me if not),
128ITSH