Datasheet

DAC161P997
www.ti.com SNAS515E –JULY 2011–REVISED OCTOBER 2013
Step Response
The transient input-output characteristics of the DAC161P997 are dominated by the response of the RC filter at
the output of the ∑Δ DAC. Settling times due to step input are shown in Typical Performance Characteristics.
Output impedance
The output impedance is described as:
(9)
By considering the circuit in Figure 17, and setting I
DAC
= I
AUX
= 0, the following expression can be obtained:
(10)
As in AC Characteristics an assumption can be made on the frequency response of the internal amplifier, and
the effective transconductance G
m
should be stabilized with external R
E
leading to:
(11)
The output impedance of the transmitter is a product of the external BJT's output resistance r
o
, and the frequency
characteristics of the internal amplifier. At low frequencies this results in a large impedance that does not
significantly affect the output current accuracy.
PSRR
Power Supply Rejection Ratio is defined as the ability of the current control loop to reject the variations in the
supply current of the companion devices, I
AUX
. Specifically:
(12)
It was shown in AC Characteristics that the I
AUX
affects I
LOOP
via the high-pass path whose corner frequency is
dependent on the effective Gm of the external BJT. If that dependence were not mitigated with the degenerating
resistor R
E
, the PSRR would be degraded at low output current I
LOOP
.
The typical PSRR performance of the transmitter shown in Application Circuit Examples is shown in Typical
Performance Characteristics.
Stability
The current control loop's stability is affected by the impedances present in the system. Figure 16 shows the
simplified diagram of the control loop, formed by the on-board amplifier and an external BJT, and the lumped
capacitances C
X1
through C
X4
that model any other external elements.
C
X1
typically represents a local step-down regulator, or LDO, and any other companion devices powered from the
LOOP+. This capacitance reduces the stability margins of the control loop, and therefore it should be limited.
RX1 can be used to isolate C
X1
from LOOP+ node and thus remedy the stability margin reduction. If R
X1
= 0, C
X1
cannot exceed 10 nF. R
X1
= 200Ω is recommended if it can be tolerated. Minimum R
X1
= 40Ω if C
X1
exceeds 10
nF.
C
X3
also adversely affects stability of the loop and it must be limited to 20 pF. C
X4
affects the control loop in the
same way as C
X1
, and it should be treated in the same way as C
X1
. C
X2
is the only capacitance that improves
stability margins of the control loop. Its maximum size is limited only by the safety requirements.
Stability is a function of I
LOOP
as well. Since I
LOOP
is approximately equal to the collector current of the external
BJT, G
m
of the BJT, and thus loop dynamics, depend on I
LOOP
. This dependence can be reduced by
degenerating the emitter of the BJT with a small resistance as discussed in Loop Interface. Inductance in series
with the LOOP+ and LOOP− do not significantly affect the control loop.
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