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DEM-ADS807E
INITIAL CONFIGURATION
By using solder switches and resistor placements, DEM-
ADS807E can be set up in a variety of configurations to
accommodate a specific mode of operation. Before starting
evaluation, the user should decide on the configuration and
make the appropriate connections or changes. The demon-
stration board comes with following factory-set configura-
tion:
• The signal input is the unbuffered input at SMA connector
J2.
• Amplifiers U5 and U6 are performing a single-ended-to-
differential conversion, using an inverting and a non-
inverting gain stage.
• The signal is ac-coupled into the ADS807E by the cou-
pling capacitors C39 and C40.
• The converter is set to operate with the internal reference.
Solder switch ‘INT/EXT’ is closed.
• The common-mode voltage required to bias the input of
the ADS807E is derived from the internal top and bottom
references by R22, R23, and R24, R25, and applied to each
signal input, pin 24 and pin 25 (U1).
POWER SUPPLY
The demonstration board requires ±5V power supply volt-
ages. Two separate power connectors are on the board.
Connector P2, labeled with –5V, GND, and +5V, is the
supply connection for the analog front end (U4, U5, U6).
Connector P1, labeled with VDRV, GND, and +5V, is the
supply connector for the converter (U1). The VDRV con-
nector is tied to pin 28 of the ADS807E. Varying the voltage
on this pin will vary the data output voltage levels accord-
ingly. Setting VDRV to +3V typically provides the best
performance results.
SIGNAL INPUT
Unbuffered Input
The factory-set configuration of the demonstration board
uses the high-speed op amp OPA642N; a voltage feedback
op amp that features low distortion. Configured in an invert-
ing and non-inverting configuration, two OPA642Ns con-
vert the single-ended signal applied to SMA connector J2
into a differential signal to drive the differential input of the
ADS807E. The signal is ac-coupled to the ADS807 and the
required level shifting is done with resistors R22 through R25
at the inputs of the ADS807E. The op amp drivers are set for
a gain of 2.
Buffered Input
The demonstration board offers the option for a second
buffered input. If this configuration is desired, several com-
ponents have to be added to the board (e.g., a buffer
amplifier such as the OPA642 and its surrounding compo-
nents). Resistor R9 (0Ω) has to be removed when wiring this
circuit configuration. The buffered input configuration has
the benefit of providing a near ideal source impedance to the
driver amplifiers, especially important for the inverting gain
stage.
Transformer Coupled
The demonstration board provides the option to evaluate the
A/D converter with either an amplifier-based differential
interface, or with a RF transformer. The RF transformer is
used to convert the single-ended input signal applied to
SMA connector J4 into a differential signal. The following
steps have to be carried out to set up the board for the
transformer coupling:
• Remove R22, R23, R24, and R25.
• Remove C39 and C40 to disconnect the op amp outputs
from the converter inputs.
• Install R27 (0Ω) to connect the common-mode voltage
available on the CM pin to the secondary side of the
transformer.
• Install R26 and R28, typically 24.9Ω.
• Add an appropriate termination resistor (R29), depending
on the selected transformer model. The installed model
has a 1:1 turns ratio.
This differential input configuration can be operated with
external references as well.
Single-Ended Operation
The flexible design of the interface circuit configuration
allows the user to operate the ADS807E with a single-ended
input signal. The following component changes must be
made:
• Disconnect the output of amplifier U5 by removing C39.
• Remove R22 and R23.
• Install R30 (0Ω).
Amplifier U6 is now ac-coupled to the IN input of the
ADS807E and is driving it single-ended. This requires the
signal swing to be twice as high as in the differential mode,
now 2Vp-p instead of 1Vp-p.
Input Full-Scale Range Select
The ADS807E provides an option for the user to select
between a 2Vp-p and a 3Vp-p full-scale input range. This
function is controlled by the logic level applied to pin 15
(RSEL) of the ADS807E. Internally, the RSEL pin has a
pulldown resistor, setting the converter up for the 2Vp-p
range. Tying the RSEL pin to a logic HIGH potential (+5V)
will switch the converter into the 3Vp-p range operation.
This can be easily accomplished on the demostration board
by closing the RSEL solder switch.