buffercircuit-designhigh-impedancemicrocontrollervoltage
How can I design a buffer circuit that takes an input of +/- 10V and scales it down to a 0V to 5V.
The idea is to design a circuit that is microcontroller compatible (can be read by an analog-in pin by a typical 5V microcontroller) with input signals from Eurorack modular synthesizers outputting +/- 10V.
simulate this circuit – Schematic created using CircuitLab
I think I need a high impedance input but don't know what kind of transistors to use or what configuration to use them in.
The biasing of a cascode is very simple ! You just apply a DC voltage to the gate of the cascoded transistor. Note that in your picture this is also the case although the gates are grounded. For small signal: Grounded = A DC voltage, remember ? Note that the cascode is a common-gate amplifier. It has current input and output.
Here's an example: M1 is the cascode, it only "buffers" the current coming out of M2 which is a common source amplifier. In this configuration M2 will be a happy NMOS since it's Vds will be almost constant. This gets rid of much of the influence of the Rout of M2. There is of course still the Rout of M1 but that does not matter as M2 determines the current, not the cascode M1 !
simulate this circuit – Schematic created using CircuitLab
LM358 is not a rail-to-rail output opamp. So it means it is not able to drive its outputs hard enough to reach the supply voltage. There will always be a voltage drop even when it saturates. This exists both on the high side and, to a lesser extent, low side.
See the datasheet to know the magnitude of this drop (which depends on the current you need on the output - the more you draw, the bigger the drop):
For the high side:
For the low side:
Best Answer
Something like this will probably work for you:
For R1 = 49.900K the ideal value of R2 would be 20.43904K and R3 would be 89.331K approximately.
The 4.096V source could be a 0.1% tolerance series reference that can sink or source current such as ADR3440. You can play with the resistor values to make them from standard values for R2/R3 as you like so long as the ratio of R1:R2:R3 is kept the same and the values are not too high or too low.
There should be a bypass capacitor on the op-amp supply and input or output protection may be required in a real circuit.
The op-amp has to be a rail-to-rail input and output type for +5V only supply.
simulate this circuit – Schematic created using CircuitLab
Note that if you leave the input open you will get an output voltage of 3.333V.