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Analog Interface
3.2
5V Supply
The ADS1675REF board requires an independent 5V supply to power the analog portion of the
device under test, the external reference, and the external reference buffer, as well as provide the
amplifier common-mode voltage. This voltage is applied to J5 pin 3 and is denoted as +5VA. This supply
can be monitored at TP7.
3.3
D5V Supply
For the best THD performance, the ADS1675 requires an independent d5V supply to power the
clock portion of the device under test. This voltage is applied to J9 pin 3 and is denoted as +5VD. This
supply can be monitored at TP3. For a quick function test, the d5VD can be shared with the analog
+5VA on this REF board. R
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is a short circuit that supplies both the analog and digital 5V to the ADS1675
device.
3.4
D3.0V Supply
For the best THD performance, the ADS1675 also requires an independent d3.0V supply to power
the digital portion of the device under test. This voltage is applied to J2 pin 3 and is denoted as +3.3VD.
This supply can be monitored at TP5. For a quick function test, the d3.0V can be fed back from the
3.3V regulator on the XEM3010 board and connected by jumper JP2 on the REF.
For optimal performance, it is recommended to remove the shunt jumpers from JP2 and JP4. This
configuration requires the user to supply an independent 3.0V digital supply to the ADS1675 at J2. The
user must also provide separate power to the XEM3010 at P1; refer to the Opal Kelly website for specific
details about configuring the XEM3010 board.
3.5
XEM3010 Board +5V Supply
To obtain the best THD performance, the ADS1675 requires an indep5V supply to power the
XEM3010 board. This voltage is applied to P1 on the XEM3010 from an external wall adapter power
supply. For a quick function test, the +5V can be shared with the d5VD on the ADS1675REF by
jumper JP4.
4
Analog Interface
The analog circuit of the board is divided into four parts. The first part is the analog input buffer or
front-end circuit of the ADC. Its function is to provide optional gain and impedance matching of the input
signal. The second part of the analog circuit is the analog power supplies for both the high voltage rails on
the ADC and the input buffer. The final sections of the analog input include the basic analog functions of
the ADS1675 and the optional external reference circuit. Each of these sections is described below.
The analog inputs to the ADS1675REF board can be applied to J1 and J3 of the input differential
connectors. The range of the analog input depends on the range setting of the ADS1675 and THS4503.
The analog input to the ADS1675REF board consists of the THS4503 operational amplifier. The THS4503
is powered from a user-provided, 9V and –4V analog supply. The amplifier is used as true differential
driver. When the analog supply is 5V, the ADS1675 analog difference input common-mode voltage is
2.5V. This configuration requires the output of the THS4503 amplifier common-mode voltage to be set as
2.5V. The common-mode voltage to the THS4503 is set by the onboard REF5025 (U1), which sets this
voltage at 2.5V.
5
SBAU162A – December 2009 – Revised September 2010
ADS1675REF
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