Datasheet

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3.4 The Output Operational Amplifier
3.4.1 Unity Gain Output
3.4.2 Output Gain of Two
EVM Operation
The EVM includes an optional signal-conditioning circuit for the DAC output through an external
operational amplifier, U2. Only one DAC output channel can be monitored at any given time for evaluation
because the odd-numbered pins (J4-1 to J4-7) are tied together. The output operational amplifier is set to
unity gain configuration by default but can be modified by simple jumper settings. Nevertheless, the raw
output of the DAC can be probed through the specified pins of the J4 output terminal, which also provides
mechanical stability when stacking or plugging into any interface board. In addition, it provides easy
access for monitoring up to two (DAC8550/51) or four (DAC8552) DAC channels when stacking two EVMs
together (see Section 3.3).
The following sections describe the different configurations of the output amplifier, U2.
The buffered output configuration can be used to prevent loading the DAC8550/51/52, although it may
present some slight distortion because of the feedback resistor and capacitor. The user can tailor the
feedback circuit to closely match the desired wave shape by simply desoldering R6 and C12 and replacing
them with the desired values. It is also possible to simply eliminate R6 and C12 altogether and to solder a
0- Ω resistor in place of R6, if desired.
Table 5 shows the jumper setting for the unity gain configuration of the DAC external output buffer in
unipolar or bipolar mode.
Table 5. Unity Gain Output Jumper Settings
Reference Jumper Position Function
Unipolar Bipolar
W3 OPEN OPEN Disconnect V
REF
H from the inverting input of the operational amplifier.
W5 2-3 1-2 Supplies V
SS
to the negative rail of operational amplifier or ties it to AGND.
W15 OPEN OPEN Disconnect negative input of the operational amplifier from the gain resistor, R12.
Two types of configurations yield an output gain of 2, depending on the setup of the jumpers W3 and
W15. These configurations allow the user to choose whether the DAC output has V
REF
H as an offset or
not.
Table 6 shows the proper jumper settings of the EVM for the 2 × gain output of the DAC.
Table 6. Gain of Two Output Jumper Settings
Referenc Jumper Position Function
e
Unipolar Bipolar
W3 Close Close Inverting input of the output operational amplifier, U2, is connected to V
REF
H for use as its
offset voltage with a gain of 2. W15 jumper must be open.
Open Open V
REF
H is disconnected from the inverting input of the output operational amplifier, U2. W15
jumper must be closed.
W5 2-3 1-2 Supplies power, V
SS
, to the negative rail of operational amplifier, U2, for bipolar mode, or ties it
to AGND for unipolar mode.
W15 Close Close Configures operational amplifier, U2, for a gain of 2 output without a voltage offset. W3 jumper
must be open.
Open Open Inverting input of the operational amplifier, U2, is disconnected from the gain resistor, R12. W3
jumper must be closed.
SLAU172A – December 2005 – Revised September 2006 DAC8550/51/52 Evaluation Module 13
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