# Flash ADC with TL074 & 74LS148: encoder inputs always high?

I am attempting to build a 3-bit 0-5V flash ADC using two TL074 quad op amps and a 74LS148 active-low priority encoder.

The circuit is fairly simple. 8 100K resistors are connected in series between 0V and +5V, with each node between connected to the inverting input of one of 7 op amps, providing 7 reference voltages for the comparators, equally spread between the supply rails. The input is connected to the non-inverting inputs of all 7 op amps. The op amps are also supplied from the 0V and +5V. Each op amp's output sits at about +1.4V until the input goes above its respective reference voltage, at which point it snaps up to about +3.6V.

My thought was that these could then directly drive the 74LS148 inputs (1 - 7, in reverse order, with input 0 grounded). If I interpret the voltages by hand according to the 74LS148's function table, I should get the (active-high) output I want. However, it seems that the inputs from the op amps are always being seen as high, even when they are at the low +1.4V state. (I do see occasional glitches where the outputs jump to a state other than all high.)

Is that +1.4V simply above or too near the logic threshold for the 74LS family? Research suggests not, but maybe I read something wrong? Or, are the output pins of the TL07x perhaps unable to sink enough current to drive the 74LS inputs low? Or is it something else?

Note that I also have -12V and +12V supplies available in this circuit, in case the op amp supply rails need to be changed.

For 74LS logic, 1.4 volts is an "undefined" value. An input must be below 0.8 volts to guarantee it will be accepted as a Low, and above 2.0 volts to be a guaranteed High.

The inputs will source 0.8 mA when taken low, so your op-amps must be able to sink that current while keeping their output voltage below 0.8 volts.

• You're absolutely right. Datasheet parse error.
– ezod
Commented Jan 17, 2015 at 20:25

You ideally need a fast rail to rail type op-amp to bring the outputs closer to the 0-5v range. Or you can use an open collector type op-amp (that guarantees pull down to 0v) with a pull up resistor on the output.

• You don't want an op amp at all. They are not optimized for fast overload recovery, and there are other problems as well. See, for instance, ronamundson.com/tag/phase-inversion for a discussion and advice. Commented Jan 17, 2015 at 23:18
• Use a comparator instead of an op amp. These can provide open-drain output, high speed, and hysteresis. Commented Jan 18, 2015 at 1:55
• What is an open collector op amp? Commented Jan 18, 2015 at 2:25
• An open collector op-amp (often just called a comparator) has the basic center circuitry of a op-amp but does not have the internal components that perform the equal output swing, it will only pull down, through an open collector. A similar mosfet version would be called an open drain type.
– Nedd
Commented Jan 18, 2015 at 11:14
• Accepted the above because it answers the question directly, but I appreciate you putting me on to this (comparators) as it appears to be the solution I want.
– ezod
Commented Jan 18, 2015 at 19:01