Datasheet

I
DAC
I
LOOP
LOOP+
LOOP-
80k 40
15k
15k 15k
390n 6.8n 220n
1n
V
HART
15 mV
1 mA
500 mV
V
H
V
H
DAC161P997
C1 C2 C3
BASE
OUT
COMA
virtual ground
500 nA
I
HART
R
E
DAC161P997
SNAS515E –JULY 2011–REVISED OCTOBER 2013 www.ti.com
Noise and Ripple
The output of the DAC is a current pulse train. The transition density varies throughout the DAC input code range
(I
LOOP
range). At the extremes of the code range, the transition density is the lowest which results in low
frequency components of the DAC output passing through the RC filter. Hence, the magnitude of the ripple
present in I
LOOP
is the highest at the ends of the transfer characteristic of the device (see Typical Performance
Characteristics).
It should be noted that at wide noise measurement bandwidth, it is the ripple due to the ∑Δ modulator that
dominates the noise performance of the device throughout the entire code range of the DAC. This results in the
“U” shaped noise characteristic as a function of output current. At narrow bandwidths, and particularly at mid-
scale output currents, it is the amplifier driving the external BJT that starts to dominate as a noise source.
Digital Feedthrough
Digital feedthrough is indiscernible from the ripple induced by the ∑Δ modulator.
HART Signal Injection
The HART specification requires minimum suppression of the sensor signal in the HART signal band (1-2 kHz) of
about 60 dB. The filter in Figure 18 below meets that requirement.
Figure 18. HART Signal Injection
RC Filter Limitation
In an effort to speed up the transient response of the device the user can reduce the capacitances associated
with the low-pass filter at the output of the ∑Δ modulator. However, to maintain stability margins of the current
control loop it is necessary to have at least C
1
= C
2
= C
3
= 1nF.
Alarm Current
The DAC161P997 reports faults to the plant controller by forcing the OUT current into one of the error bands.
The error current bands are defined as either above 20 mA, or below 4mA. The error band selection is done via
the ERRLVL pin. The exact value of the output current used to indicate fault is dictated by the contents of
ERR_HIGH and ERR_LOW registers. See ERR_LOW and ERR_HIGH.
The default settings for LOW ERROR CURRENT and HIGH ERROR CURRENT are specified in ELECTRICAL
CHARACTERISTICS
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