Electronic – Op amp output resistor value

One parameter of the opamp you are violating is the input common-mode range.

For the LM358 and many other opamps the inputs must not be with a certain voltage of the positive or negative supply rail.

The LM358 was one of the first that allowed the inputs to go down to the negative rail and is often referred to as a single supply opamp. Even the LM358 does not allow the inputs to get closer to the positive supply rail than 1.5V, i.e. 3.5V if you are operating off 5V. If you supply the amplifier with not less than 6.5V it would function. The output would then be 0.2V.

Some opamps have what are called rail-to-rail inputs and do allow the inputs to go to the positive rail.

Another way if you are constrained to a 5V supply is to supply the non-inverting input with a voltage such as 2.5V. That will allow the opamp to function but with a 48uA input current and 100k feedback the output will attempt to go to -2.3V which is outside the output stage capability since you are only supply it with 0 and 5V. You could give it a negative supply (say -5V) or reduce the 100k resistor to 50k for example. The output voltage would then be 2.5V (the input common mode) - 48uA*50K (the voltage developed across the feedback resistor 2.4v) = 0.1V at the output.

The series resistor to the non-inverting input serves no function as the current into the opamp is so small (~25nA) that it will not change the voltage. If you connect the 100k variable as a potentiometer between 5V and ground it would allow you to adjust the input voltage.

What you're seeing is a mismatch of the turn-on/turn-off time of the msb compared to the other bits.

Pretend for a moment that this is an 8-bit DAC, sitting at code 01111111. If the next code were 1000000, you'd get a well-behaved step. But what's happening is that internally the msb responds a bit quicker than the other bits. This means that the transition is actually 0111111 to 1111111 to 10000000. The 11111111 code only persists for a very short time, but while it's there the output tries to go to full-scale. Since the mismatch time (called the bit skew) only lasts for a very brief time you just get a relatively small pulse (commonly called a glitch). This effect is actually quite important in video DACs, since the eye is very sensitive to this sort of thing, and an important characteristic for video DAC is total glitch energy.