August
1998, Issue 97
BitScope
A Mixed-Signal Capture Engine
WALKING
THRU SCHEMATICS
Before
delving into the schematics, take a look at Figure 1,
which overviews the functionality of the BitScope design.
Figure 1—This block diagram
of the mixed-signal capture engine shows basic
design architecture.
|
The
PIC, the Lattice PLD, and the SRAMs are shown in Figure
2a. These chips are closely coupled to form the sample
capture functions at the core of this design.
| Figure
2a—The BitScope CPU and
storage engine includes the PIC, PLD logic, clock,
data muxes, and sample RAM. |
By
using a synchronous tristate clocking circuit, the PIC
is able to stop, start, and preload the Lattice PLD
using just a handful of signals. Notice that it’s
necessary to read in data from the RAM chips one bit
at a time because there are no spare eight-bit ports
available.
One
fundamental rule in mixing analog and digital circuits
is to avoid contamination of the analog grounds. Figure
2b shows that great care was taken to isolate the analog
and digital sections of this circuit at high frequencies.
Similarly with the RS-232 port, it’s best not to
allow PC noise to have any path to a test circuit.
| Figure
2b—BitScope power supply and comms
deal with filtering, rectification, and regulation
as well as RS-232 level shifting and indicators.
|
Digital
test signals and two spare analog signals are shown
on Figure 2c connecting to the DB25M pod connector.
Logic levels are latched and conditioned ready for storage
in the digital sample RAM.
| Figure
2c—The BitScope digital capture
unit has a logic pod circuit with latching buffer
and pod I/O switches. |
You
might guess from the extra signals on the pod that it’s
not just eight logic levels in. As well as fused balanced
power supplies, there is a digital I/O communication
port. Everything you need is there to connect an active,
programmable extension module.
Most
of the analog conditioning circuits and the flash ADC
are shown in Figure 2d. The circuit consists of an amplifier
chain driving through a pair of 4:1 analog mux devices.
| Figure 2d—The
BitScope analog capture features the vertical
channel muxes, attenuation switch, ADC buffer,
and ADC. |
Modern
video op-amps help here. They give you high input impedance,
low output impedance, and unity gain stability.
The
PIC controls the mux sources that allow implementation
of range switching and channel chop functions. To accommodate
different ADC chips, there are adjustment pots for both
the range and offset voltages as required by the manufacturers.
Figure
2e shows the final part of the analog conditioning circuit.
Channels A and B are standard 1-MB input impedance AC/DC
BNC connectors. A classic source follower tree driving
a unity gain buffer for each channel completes the vertical-amplifier
section.
| Figure
2e—The BitScope Input Channel
Buffers are high-impedance voltage followers and
op-amp buffers with a 1-GHz prescaler circuit. |
For
engineers who like to measure high frequencies, I added
a small 1-GHz prescaler circuit, which includes a switchable
50-W terminator hanging off the Channel B input circuit.
Note that BitScope has a couple of ways to measure the
frequencies of applied signals. I explain the motivation
behind this in the sidebar "Subsampling—Bending
Nyquist.
