Hi,
I'm testing an 8-bit R2R ladder based D/A converter and I can't get it to work properly. This will be a general purpose converter to be used with an ATmega16 development board. The resistor ladder itself works OK; the output voltage goes in even steps from 0 to +5V, even the 01111111 - 10000000 step is not bad because I carefully measured and picked those resistors to minimize uneven steps.
However, the voltage follower opamp works only from 0 to about 3.7V, but any input voltage above that makes the output to be 5V. The opamp is LMC660 which is said to be rail-to-rail, but is it really so? I think the problem would be solved by having at least 6.5V supply for the opamp (and no need for rail-to-rail then), but there's no more than 5V available. Also, I don't want to reduce the output range to overcome that problem. Is there any other opamp which would really work up to +V, or some other ideas?
I'm testing an 8-bit R2R ladder based D/A converter and I can't get it to work properly. This will be a general purpose converter to be used with an ATmega16 development board. The resistor ladder itself works OK; the output voltage goes in even steps from 0 to +5V, even the 01111111 - 10000000 step is not bad because I carefully measured and picked those resistors to minimize uneven steps.
However, the voltage follower opamp works only from 0 to about 3.7V, but any input voltage above that makes the output to be 5V. The opamp is LMC660 which is said to be rail-to-rail, but is it really so? I think the problem would be solved by having at least 6.5V supply for the opamp (and no need for rail-to-rail then), but there's no more than 5V available. Also, I don't want to reduce the output range to overcome that problem. Is there any other opamp which would really work up to +V, or some other ideas?