AD9280: reference span

Document created by analog-archivist Employee on Feb 23, 2016
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We want to use the AD9280ARS in a framegrapperapplication. The
opportunity is 4k/a.
He has some questions.

Is it possible to make the difference inputvoltage smaller than 1Vpp, for
example with a smaller reference etc, or is the min. inputrange 1Vpp?

What is the difference between the REFTx and REFBx. When should the cst. drive
VREF without the internal Amplifierer, and when should he use the REFT/Bx-pins?
Can you pls. describe this a little bit more detailed whats happen with the
referencevoltage?

 

The minimum recommended reference span is 1V. It is possible to use the AD9280
with a reference less than 1V by using an external reference to drive REFTS and
REFBS but the Signal to Noise Ratio will be reduced since you will have reduced
the full-scale input signal but the noise will remain the same.

REFTS: REFerence Top Sense
REFBS: REFerence Bottom Sense
REFTF: REFerence Top Force
REFBF: REFerence Bottom Force

There are number of different reference modes available on the AD9280, The
particular reference mode is selected by tying the MODE pin to a particular
voltage. The MODE pin controls internal switches (which are not shown in the
datasheet) in order to select the particular reference mode.

1. With MODE connected to AVDD, Top/Bottom mode is selected (see figure 18). In
this mode REFTS and REFBS are the inputs to the reference buffer amplifier,
REFTF, REFBF are the outputs of the buffer amplifier which should be decoupled
as shown in the datasheet, this is the actual voltage which is applied to the
ADC core.

2. With MODE connected to AVDD/2, center span mode is selected (see figure 20).
In this mode REFTS and REFBS are disconnected from the reference buffer and
instead act as a second differential input to the SHA. The reference buffer is
driven directly from the Vref pin.

3. With MODE connected to AGND, the part operates in "AD876 mode". The
reference buffer is taken out of the circuit and an external reference is used
to drive the reference to the ADC core directly through REFTF and REFBF. The
AD9280 should be used in this mode if it is being used as a "drop in"
replacement for the AD876. 

Most customers will use one of the first two modes described. For best
distortion performance, drive the part differentially in center span mode with
a reference of 1Vpkpk. Best distortion performance is achieved when the input
is centered around mid-supply and full-scale voltage is kept small.

For best noise performance, use the 2V reference to give maximum full-scale
input.

An external reference is recommended when the main requirements are for DC
precision and low thermal drift.

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