Can anyone provide some information on how the internal arbitrary waveform generator of the more mature Rigol Oscilloscopes is supposed to be calibrated? Several years ago, I acquired a DS2072A-S from Rigol Europe's Clearance Center. Since for what ever reason it contained the MSO hardware, I turned it into an MSO2302A-S. Only later I found out that the AWG section apparently has got a problem with a DC offset of about -150mV if the output level is increased above 799mVpp 50Ohms / 1.59Vpp / hi-Z on both channels (ch1 is a little worse than ch2).
Recently, I had a closer look at the AWG board to analyze if the offset may be a hardware problem, but while I found some peculiarities, it apparently isn't. FYI, here are photos of the really tiny AWG module and some information on its technical details:
Main semiconductors:
FPGA Altera Cyclone IV EP4CE22F17C8N
D/A Converters AD9744
Aux D/A Converter DAC8560
Transimpedance amps / pre-drivers: AD8009
Output amps: LMH6702
Offset voltage amp: AD8512
First thing that I noticed was the somewhat "roasted" appearance of ch2's 50R output resistor. Even though it must have been overloaded at some time, it's resistance is still spot-on and the output amp is also performing well. The output overvoltage protection (LM339) should have taken care of that but maybe some too low voltage to trigger the protection had been applied to the output for considerable time so the 1210 resistor just overheated somewhat. This must have happened before I received the instruement, probably while it spent its "first life" as a demo unit...
Moreover, I found an edge of the LM339 chipped, which happened probably during assembly since there's a threaded stud located next to it. This shouldn't really matter, though...
The final output amps are only utilized at output voltages above ~1.6Vpp (high-Z) or ~800mVpp (50Ohm Load), otherwise they are bypassed by the second relays in the rows. The rightmost relays enable the generator outputs while the leftmost ones select between a 20dB / 63.7 Ohm attenuator or a straight-through. The pre-drivers's output series resistance is 63.2 ohms and thus matches the attenuator. And this is the interesting part: The offset is generated by a separate 16 bit D/A converter, amplified by an opamp to quite a high level and then resistively added to the main output signal via the 249 Ohms resistors located on the bottom of the PCB. Since the load "sees" the 63 Ohms and the 249 ohms in parallel, the effective output resistance is ~50 Ohms.
This way, it's possible to add a decent range of offset without having to sacrifice resolution of the AC output signal. On the down side, the offset output amp has to be run at quite a considerable operating voltage, in this case +-15V. The offset voltages and two more internal DC control voltages are generated via a single-channel DAC8560 D/A converter, and sampled via HC4051 analog multiplexer to four channels to chare small storage caps, connected to a TLC274 CMOS amplifier that acts as a buffer.
Altogether, the design is quite straight-forward and the compopnent choice is decent. Since there's no non-volatile memory present on the AWG module, all the calibration has to be done via and stored in the host oscilloscope. If I only knew how that's supposed to be accessed...
. Any help would be greatly appreciated
.
Cheers,
Thomas