Datasheet

DAC
-+
I
DAC
I
AUX
I
LOOP
LOOP+
LOOP-
R
1
= 80k
+
-
I
D
I
A
BASE
OUT
COMA
R
2
= 40
I
E
I
2
DAC161P997
VD
VA
R
E
DAC161P997
www.ti.com SNAS515E –JULY 2011–REVISED OCTOBER 2013
APPLICATION INFORMATION
16-BIT DAC AND LOOP DRIVE
DC Characteristics
The DAC converts the 16-bit input code in the DACCODE register to an equivalent current output. The ∑Δ DAC
output is a current pulse which is then filtered by a 3
rd
order RC low-pass filter and boosted to produce the loop
current I
LOOP
at the device OUT pin.
Figure 13 shows the principle of operation of the DAC161P997 in the Loop Powered Transmitter - the circuit
details were omitted for clarity. In this figure I
D
and I
A
represent supply (quiescent) currents of the internal digital
and analog blocks. I
AUX
represents supply (quiescent) current of companion devices present in the system, such
as the voltage regulator and the SWIF channel.
By observing that the control loop formed by the amplifier and the bipolar transistor forces the voltage across R
1
and R
2
to be equal, it can be shown that, under normal conditions, the I
LOOP
is dependent only on I
DAC
through
the following relationship:
(1)
While I
LOOP
has a number of component currents, I
LOOP
= I
DAC
+I
D
+I
A
+I
AUX
+I
E
, it is only I
E
that is regulated by the
loop to maintain the relationship shown above.
Since it is only I
E
’s magnitude that is controlled, not its direction, there is a lower limit to I
LOOP
. This limit is
dependent on the fixed components I
A
and I
D
, and on system implementation through I
AUX
.
Figure 13. Loop-Powered Transmitter
Figure 14 shows the variant of the transmitter where the supply currents to the system blocks are provided by the
local supply, and not the 4 - 20 mA loop Self-Powered Transmitter. Same basic relationship between the I
LOOP
and I
DAC
holds, but the component currents of I
LOOP
are only I
DAC
and I
E
.
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