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Chapter 3, TECHNICAL DESCRIPTION
3.3.21 Main Output Processor - Standard Curve Buffer
3.3.22 Analog Outputs (DACs)
3-14
control is enabled, then the response will be -1. Since noise readings can only be zero
or positive, this negative number clearly indicates that the reading is invalid and
should be ignored.
In order to make noise measurements easier, the instrument includes a noise
measurement mode, activated by a control on the Configuration menu or by a
computer command. When this is turned on, the Main Display outputs are set to the
four types most commonly required, and the filter time constant, slope and
synchronous time constant setting are forced to values within the permitted ranges.
When turned off, these restrictions are removed.
When making noise measurements the user is strongly advised to use an oscilloscope
to monitor the signal at the SIG MON output on the rear panel as this is the best way
of ensuring that a random process is being measured rather than line pick-up or other
non-random signals.
The output processor also operates the standard curve buffer, which is a 100,000
point memory that can be used for storage of selected instrument outputs as curves,
prior to their transfer to a computer via the computer interfaces. In addition to using
this function for the instrument outputs, such as the X channel and Y channel output
signals, it may also be used to store derived outputs and reference frequency
information.
Unlike the fast curve buffer, the available points are split between the number of
outputs to be stored, so that if for example X1 and Y1 outputs were selected, the
maximum recording length would be 50,000 points. Storage operates at rates of up to
1 kSa/s (1 ms per point).
Earlier SIGNAL RECOVERY DSP lock-in amplifiers provided two different types
of analog outputs. Typically there were two outputs driven by DACs that in turn
were connected to the main instrument outputs, and separate auxiliary outputs,
essentially simply programmable DC voltages typically used as control signals for
the experiment. However this architecture created a number of restrictions. In
particular, in the dual modes it was not possible to get both X and Y outputs for both
demodulators when using short time constants, and in some cases it was necessary to
take the analog output from different connectors for different time constant ranges.
This design has been updated and so the 7270 is fitted with four general-purpose
DAC outputs, which can be driven from a variety of output signals, as well as the
traditional programmable "auxiliary DAC" signal, now referred to as the "User
DACs".
Selection of the required outputs for the DACs is made on the DACs Menu. For each
DAC output there is a selection control, which is used to choose which instrument
output will be sent to the relevant DAC, or to specify that it will be a User DAC. If
the latter is selected then a further control allows the DAC voltage to be set.
Each DAC output is driven by a 16 bit converter operating at a raw update rate of
1 MSa/s, although depending on the output it is generating the actual update rate is
either 1 MSa/s or 1 kSa/s. When used for instrument outputs, full scale corresponds
to ±2.5 V, but the output remains linear to up to ±7.5 V; when used as a User DAC,
the range is ±10.000 V.
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