The Case of the Dancing Bandgap

A few years ago, I contributed to the design of a mixed signal IC for an automotive customer.

The IC was an electrochromic mirror driver.  Self-dimming rearview mirrors are common place in most automobiles. In addition to a shunt voltage regulator for controlling the voltage across the mirror element, there were oscillators, ambient light sensors (ALS) and a bandgap voltage reference.  When the ALS experiences a high light input, the voltage across the electrochromic mirror driver reduces the glare as seen by the driver of the vehicle.

When the test chip arrived from the fab, I set about testing the various functions.  The IC was placed in test mode which would allow the bandgap voltage to be routed to a pin during evaluation. The bandgap voltage reference operated as expected.  Next, the oscillator was probed and displayed on the oscilloscope.  The 1 MHz clock signal looked as expected. The duty cycle was about 50 percent and negligible ringing was observed.  I had left the oscilloscope probe connected to the bandgap signal.

As I disconnected the oscillator probe, I noticed a change in the bandgap voltage waveform. When I reconnected the oscillator probe, there was a noticeable decrease in the bandgap voltage waveform.  This effect was totally unexpected since there should be no electrical connection between the two functional circuit blocks.  The bandgap reference was not used in the oscillator circuit.

Upon inspection of the layout of the chip, a small, almost unperceivable routing issue was observed.  The routing for the bandgap reference and oscillator were hundreds of microns apart.  However, upon closer inspection, it was observed that there was kink in the oscillator trace routing.  There was about 10 microns of trace metal that was a few microns away from and parallel to the bandgap trace.

Why should this matter?  The typical probe pad capacitance was 50fFarads. When the oscillator is loaded with a 25pF probe, there is a 500X decrease in load impedance.  The impedance change resulted in a significant increase in transient current.  The magnetic fields surrounding the oscillator trace increase with increasing load capacitance. This transient field inductively couples signal onto the bandgap voltage via the closely space traces resulting in a change in the voltage.  This was observed on the oscilloscope.