## Designing a Split Termination

If no suitable voltage source exists for a relatively simple end-terminating structure then you have no choice: You must synthesize the Thevenin equivalent.

My last two articles dealt with the design of the
end-terminating structure in Figure
1a ([1], [2]). That structure
terminates a transmission line with a single resistor value, R_{T} (termination resistance), leading to a fixed V_{T} (terminating
voltage). In most cases, the termination resistance equals the characteristic
impedance of the transmission line or can be a little higher than that impedance
if the driver is weak. The ideal terminating voltage centers the digital
waveform, producing equal voltage margins above and below the required switching
levels, V_{OH} (high output voltage) and V_{OL} (low output
voltage). That ideal voltage equals the average of the required high- and
low-output levels minus a correction term that accounts for asymmetry in the
drive capability of the source:

This equation assumes that the I_{OH} (high-side source
current) is positive and the I_{OL} (sinking current) is negative. If
those two values have equal magnitudes, they sum to zero.

If you have a suitable source of terminating voltage, the structure in Figure 1a is easy to understand and uses fewer components than the one in Figure 1b. On the other hand, if no suitable voltage source exists, then you have no choice: You must synthesize the Thevenin equivalent from Figure 1b.

Given any combination of termination resistance, termination voltage, and
V_{CC} (power-supply voltage) with voltage greater than the termination
voltage, the following circuit values make the circuit in Figure
1b perform, from the transmission line's perspective, just as well
as the one in Figure 1a.

If you carefully follow the equations, you are now almost finished with your design. Next, you will discover that, no matter what values you have just computed, those exact values are never available in your component catalog. The values in the catalog are quantized to the nearest standard value, according to the tolerance specification for each component.

If you want your circuit to perform over a range of resistor values and over
a range of possible values for the power-supply voltage, check the worst-case
constraints in Figure 1. If resistor R_{1} meets these conditions, then the
circuit in Figure 1b will work under all conditions just as well
as the one in Figure 1a. Make sure that the minimum and maximum
values take into account temperature and aging. Some fiddling with the values
will be necessary; there is no straightforward design approach that always
works.

If you have difficulty satisfying the constraints, try raising your target
for the termination resistance and start again. Although it will not terminate
the circuit as well, increasing the termination resistance opens more room for
tolerance in the circuit. The tolerance requirements for R_{1}and R_{2} are somewhat interchangeable. A tighter tolerance for R_{2} opens more room for R_{1}and vice versa.

I cannot help you further with this aspect of the design. Component selection in analog circuits always involves some last-minute juggling to meet all the tolerance requirements. I can, however, point out that the circuit in Figure 1a helps you understand the need for two resistor values and how they work together to meet the impedance and current-drive constraints your driver imposes.

### References

**[1]** Johnson, Howard, PhD, "Yao!
What a handshake!" *EDN*, Feb 7, 2008, pg 22.

**[2]** Johnson, Howard, PhD, "Z[MIN], a very special
value," *EDN*, March 6, 2008, pg 24.