USB Large capacitance on data lines

Summary

This post details how the fitting of incorrect capacitors on USB lines can result in operational issues.

Hardware

After several years of use some of the surface mount capacitors on my FTDI branded debugger module showed signs of cracking, time to replace them.

FTDI VNC2 Debugger Module

Both capacitors (C8 & C9) associated with the USB D+ and D- lines were removed without any resulting communications issues. Schematic extract below was taken from the VNC2 Debugger datasheet.

FTDI VNC2 Debugger USB Input Section

As the USB line capacitors are there for multiple reasons, replacements were located. FTDI recommend capacitors in their Debugging Application Notes to prevent the reset of FTDI devices from spurious signals.

Wrongly Marked Components

The two USB line capacitors were replaced with what was presumed to be similar devices. Upon plugging the debugger into the computer an error message was displayed stating that the USB device could not be identified.

USB Signals

To diagnose further the oscilloscope was powered and the two USB signals probed.

USB Signals with Large Capacitors

The capture above shows the signals. Excessive rounding of the waveform usually indicates a high capacitance. 

Removing the new capacitors and verifying their capacitance confirmed that the value on the manufacturer packaging was incorrect.

USB Signals with Small Capacitors

With the capacitors removed the debugger began operating normally. Alternative replacement were fitted. The subsequent capture shown above is more indicative of USB signals.

Rigol DP832 Output Capacitor Limitations

Summary

This blog details a hindrance with the Rigol DP832 power supply outputs when used for purposes such as characterising digital inputs, circuit reaction times or external power supply transient performance to name handful.

Rigol DP832

DP832 'Gotcha'
The Rigol DP832 is a good mid-range power supply sporting an easy to use interface to suit the hobbyist and options such as remote communications and power cycling to cater for some professional applications. For engineers familiar with power supply interruption testing, as defined in EN61496-1, the Rigol DP832 can be setup, with some fiddling, to assist with compliance tests.

It was the power cycling feature on the DP832 that was to be used for testing the PLC input design, however after the first few power cycles some unexpected measurements were observed. The configurable PLC input section circuit is shown below.

PLC Input Section

The power On captures shown below illustrate the difference between using the Rigol On/Off power button and the second capture shown the response using an external switch with the Rigol power button set to On.

Power ON Delay using Rigol Power Button

Power ON Delay using External Switch - Rigol Power ON

The delay in powering the circuit Off was also easily visible on the scope.

Power OFF Delay using Rigol Power Button

Power OFF Delay using External Switch - Rigol Power ON

Output Capacitance
Looking at the captures from the scope this appeared to be output capacitance. I recalled that Dave Jones from EEVBlog had torn down the DP832 several years ago; the output capacitors where found in EEVBlog #511; credit and many thanks to Dave Jones saving me the teardown.

The output capacitors appeared to be 1000uF electrolytics, which would explain the delayed switching performance.

EEVBlog #511 DP832 Output Capacitors - 2 Channels

EEVBlog #511 DP832 Output Capacitors - 1 Channel

Output Solution
As a temporary solution to testing, a relay was added. Channel one performed the relay coil switching and Channel two was left powered with the output connected to a Normally Open relay contact. The switching response was did not have the effect of the capacitor charge time although some relay contact bounce was evident.