Design Con 2015

Boost and modify square waves

-July 14, 2014

TTL and CMOS
The easiest and the least expensive way to build an EPM is to use standard TTL and CMOS integrated circuits as buffers between the SQWG and the load.

We have large variety of TTL and CMOS buffers with totem-pole outputs, open collector/open drain outputs, and three-state outputs—74xx125, 74xx126, 74xx241, 74xx540, 74xx541, etc. Although the buffers with the same main number (e.g. 74xx540) and with different technologies are pin-to-pin compatible and even functionally compatible, they may have significant differences in their AC and DC characteristics, especially in the power supply range and the diving capabilities.

Figure 2 shows a dual-channel EPM with 74xx540 (inverting) or 74xx541 (noninverting) outputs. The eight channels are separated in two groups that work in the following configurations:

  • Independently as two groups with four buffers.
  • The second group after the first group or the first group after the second group.
  • All eight inputs can be put in parallel and ideally we will have the same signal at the all eight outputs.


Figure 2. Circuit diagram of simple dual channel EPM with 74xx540 or 74xx541 with total 8 (2x4) outputs.

When designing with TTL, pay attention about the driving capabilities and the output levels of the SQWG. Most of the SQWGs can drive more than 10 standard TTL inputs, but the output characteristics should be checked before usage.

Some of the SQWGs can add DC offset output to a signal or even generate a bipolar square signal. The outputs of the 74xx540 and 74xx541 have three states. We can use the signals /E1 and /E2 to enable or disable all eight outputs in the same time. The series resistors at the inputs and outputs of the buffers are required, but the exact values depend on the buffer technology and the type of load. We also add diodes at the inputs and the outputs to protect the buffers from over voltages and under voltages.

Digital ICs with eight buffers such as the 74xx240, 74xx241, 74xx540, 74xx541 and similar devices give us opportunities of building inverting and noninverting EPMs. These ICs have Schmitt-trigger inputs and can be used with signal generators producing signals with non-TTL/CMOS parameters.

Figure 3 shows a four-channel EPM using 74xx540 or 74xx541. The module has 16 outputs operating in four groups with four buffers per group. Some of the buffers can be used with paralleled outputs with appropriate equalization resistors.

Figure 3. Block diagram of simple four channel EPM with 74xx540 or 74xx541 with total 16 (4x4) outputs.

Sometimes we need to automatically or manually enable or disable the buffer outputs to put them in tristate mode. In these cases, we use buffers such as the 74xx125 or 74xx126. Figure 4 gives the circuit diagram of a four-channel (4x1) EPM with a 74xx125 or 74xx126, where individual outputs enable or disable each output buffer.

Figure 4. Circuit diagram of simple four channel (4x1) EPM with 74xx125 or 74xx126 with an individual output enable for each buffer.

Next: TTL and CMOS

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