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ADC will be 11111111 (255 decimal). If the analog input level to the ADC
is equal to 0.0 V, then the output of the ADC will be 10000000 (128
decimal).
To obtain the highest resolution and accurate measurements, the input to
the ADC must be scaled within its dynamic range, which is ± V. Although
the ADC has a limited and fixed input dynamic range based on the ADC's
reference voltages (± V), scopes must be able to capture a broad dynamic
range of signals including both high level and low level input signals.
Scaling the ADCs analog input to be within the ADC's dynamic range is
the combined function of the attenuator, DC offset, and amplifier blocks,
which are discussed next.
Attenuator Block
The attenuator block is basically a network of resistor dividers used to
scale the input signal to be within the dynamic range of the scope's
variable gain analog amplifier and ADC. When inputting a high level input
signal such as 40 Vpp, the signal level must be reduced (attenuated). If
inputting a low level input signal such as 10 mVpp, then the input signal
is passed through to the amplifier with no attenuation (1:1). When you
change the scope's V/div setting, you may hear audible clicking sounds.
The clicking that you hear are mechanical relays switching- in different
resistor divider networks. Note that the attenuator block also includes
switching of the user- selectable input impedance (1 M
as AC or DC input coupling.
DC Offset Block
When inputting a signal with DC offset such as a digital signal that swings
between 0V to 5V, if we want to display this signal centered on the scope's
display, then an internal DC offset of the opposite polarity must to added
to the signal in order to shift the input signal to be within the ADC's
dynamic range. Alternatively, AC coupling can be selected to eliminate the
DC component of the input signal.
Amplifier Block
The final analog processing stage to scale the input signal to be within the
dynamic range of the ADC system is the scope's variable gain amplifier. If
inputting a very low level input signal, you would typically set the V/div
setting to a relatively low setting. With a low V/div setting the attenuator
stage would then pass this signal straight through to the amplifier with no
attenuation (gain =1), and then the amplifier would boost (gain >1) the
amplitude the signal in order to take advantage of the full dynamic range
2000 X-Series Oscilloscopes Advanced Training Guide
Oscilloscope Block Diagram and Theory of Operation
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