Terminating Digital Pattern Instruments

Contents

Terminating Digital Pattern Instruments

You can use high-impedance load termination or 50 Ω load termination to terminate the pin driver of the digital pattern instrument. You can use the high-impedance comparator configuration or 50 Ω comparator termination to terminate the comparator of the digital pattern instrument. 

 

Pin Driver Termination

Refer to the following table to determine the recommended load termination for your system.

Source (Digital Pattern Instrument Pin Driver) Transmission Line Transmission (at DUT)
ZS = 50 Ω ZO = 50 Ω

High-impedance termination

Zt = High-impedance (>10 kΩ)

Most common load termination. Causes reflections but preserves the majority of the voltage swing at the load.

50 Ω termination

Zt = 50 Ω

 

High-Impedance Load Termination

A common configuration for digital pattern instruments is to connect the output terminals of the instrument directly to the DUT. If the digital pattern instrument is connected to a high input impedance DUT, the configuration is referred to as high-impedance load termination.

When using high-impedance load termination during DC operation, minimal voltage drop occurs across the source impedance of the digital pattern instrument due to low current flow in the high-impedance signal path. When using high-impedance load termination during AC operation, the signal reaches maximal amplitude at the DUT.

Digital edges reflect off the high input impedance DUT and propagate back towards the digital pattern instrument. If you match the transmission line to the 50 Ω input impedance of the instrument, the reflections will be absorbed at the instrument and will not impact signal integrity. If you poorly match the transmission line to the 50 Ω input impedance of the instrument, additional reflections will be generated in the transmission line and will negatively impact signal integrity.

 

50 Ω Load Termination

When connecting the output terminals of the instrument directly to a 50 Ω input impedance DUT, the configuration is referred to as 50 Ω load termination. You can achieve a 50 Ω load termination for a high input impedance DUT by placing a 50 Ω resistor on the signal trace at the end of the transmission line near the DUT pin. Digital edges terminate at the 50 Ω load and no reflections propagate back towards the digital pattern instrument. 

Populating a 50 Ω termination at the DUT may help with some reflections in the system, but will also add loading under all conditions, including PPMU operations. Adding the termination will make it impossible to perform some tests, such as leakage measurements. Use high-impedance termination or provide mechanisms to switch out the termination if you perform these types of PPMU tests.

 

Comparator Termination

Refer to the following table to determine the recommended comparator termination for your system. 

Source (DUT) Transmission Line Termination (at Digital Pattern Instrument)
ZS = 50 Ω ZO = 50 Ω Zt = high-impedance

ZS ≠ 50 Ω

The configuration is not recommended because of the mismatch on both ends. The recommended setup is to increase ZS to 50 Ω by adding serial resistance at the source.


Note
In order to use a source impedance other than 50 Ω, you might need to reduce the operating frequency to allow the signal reflections to settle down. You must reduce the frequency proportional to cable length and impedance mismatch.

ZS = 50 Ω

Vi = Vo /2 where Vi is the pin input voltage at the digital pattern instrument and Vo is the DUT driver output voltage.
Zt = 50 Ω

Source must be able to drive 50 Ω load.

ZS ≠ 50 Ω

Source-side mismatch degrades signal integrity, but this configuration is still usable in many applications.

Vi= 50/(50 + ZS) x VO

 

High-impedance Comparator Termination

Connecting the output pin of a DUT to the high input impedance of the digital pattern instrument is referred to as high-impedance comparator configuration. You can set the digital pattern instrument to have high input impedance by configuring any pin for a termination mode of High Z and driving an X on the pin. For the comparators, the instrument is the termination and the DUT is the source. You should match the output impedance of the DUT to the transmission line connecting the DUT to the digital pattern instrument. Typical configurations use a 50 Ω source impedance within the DUT matched to a 50 Ω characteristic impedance cable. Digital edges reflect off the high input impedance of the instrument and propagate back towards the output pin of the DUT. If the source impedance of the DUT is not well matched to the characteristic impedance of the cable, additional reflections continue to propagate back and forth between the source impedance of the DUT and the termination impedance of the digital pattern instrument. To maintain signal integrity at fast vector rates, match the DUT source impedance to the characteristic impedance of the cable to properly terminate any reflections caused by the high input impedance of the instrument.

 

50 Ω Comparator Termination

Connecting the output pin of a DUT to the 50 Ω input impedance of the digital pattern instrument is referred to as 50 Ω comparator termination. You can set the instrument to 50 Ω input impedance by configuring any pin for a termination mode of VTERM, setting the VTERM pin level to a voltage within the specified range of the instrument, and driving an X on the pin. For the comparators, the digital pattern instrument is the termination and the DUT is the source. You should match the output impedance of the DUT to the transmission line connecting the DUT to the instrument. The output impedance of the DUT and the characteristic impedance of the cable should be 50 Ω, which matches the 50 Ω input impedance of the terminating pin of the instrument. In this configuration, digital edges terminate at the 50 Ω input impedance of the digital pattern instrument, and no reflections propagate back towards the source impedance of the DUT. You can achieve high signal integrity with the 50 Ω comparator termination configuration, however, the configuration affects requested signal amplitude by cutting it half. You must set comparator thresholds according to the reduced voltage caused by the termination.