Datasheet
Digitally Programmable Current Source Application
3-5
EVM Operation
Table 3−3.Unity Gain Output Jumper Settings
Jumper Setting
Reference
Unipolar Bipolar
Function
W15 OPEN OPEN
Disconnect the inverting input of operational amplifier,
U2, from AGND.
W5 2−3 1−2
Negative rail of operational amplifier is tied to AGND or
powered by V
SS
.
3.4.2 Output Gain of Two (Default Configuration)
Table 3−4 below shows the proper jumper settings of the EVM for the
2x gain output of the DAC.
Table 3−4.Gain of Two Output Jumper Settings
Jumper Setting
Reference
Unipolar
Bipolar
Function
W15 CLOSED CLOSED
Inverting input of the output operational amplifier, U2,
is connected to AGND to set for a gain of 2.
W5 2−3 1−2
Supplies power, V
SS
, to the negative rail of operational
amplifier, U2, for bipolar supply mode, or ties it to
AGND for unipolar supply mode.
3.5 Digitally Programmable Current Source Application
A digitally programmable current-source circuit is added for the convenience
of the users. Any DAC channels of the DAC7654 can be selected to generate
the voltage output that can be connected to the external operational amplifier,
U2. The external operational amplifier is used to perform this operation
because of the closed-loop configuration, internal to the device, of each
DAC7654 output amplifier. Therefore, the external operational amplifier, U2,
is used to drive the transistor for this type of operation. The selected transistor
(Q1 or Q2) is placed within the loop (i.e., U2 must be configured for open-loop
gain) to implement a digitally programmable, unidirectional current source, as
shown in Figure 3−2.
To operate the DAC7654 for the digitally programmable current source
application, the DAC7654 must be configured for the unipolar mode of
operation. The DAC7654 channels A and D are the only ones shown in the
table, but any DAC channel works.
The resistor R6 and the capacitor C12 should be disconnected from the circuit
of U2 for open-loop configuration. The resistor R4 can be replaced with a 0-Ω
resistor or a jumper wire. An extra wire is needed to connect TP5 to W22−2
or W24−2, as well as the J5−1 to W11−2 or W21−2.
See Table 3−5 for the setup configuration.