Circuit Note CN-0278
Measurements were also completed whereby the slew rate was
reduced even further by changing the SR clock setting to 5
rather than 3, and leaving all other settings and component
values unchanged, the effects of which can be seen if Figure 9
and Figure 10 are compared.
Noise During Silence Measurements—AD5422 TSSOP
Extra measurements were also taken in an effort to simulate
the behavior of the AD5422 TSSOP package option in this
configuration; however, without the capacitor on the CAP1 pin
(C1) present (because the TSSOP version of this part does not
contain a CAP1 pin).
While the results for output noise during silence tests were greater
without C1 in place, than in the case of the LFCSP part with
C1 in place, they were still within the HART Communication
Foundation protocol specifications. Channel 2 in Figure 12
and Figure 13 shows the broadband noise results with the
HCF_TOOL-31 filter in place, 530 µV rms for 4 mA I
OUT
and
690 µV rms for 12 mA I
OUT
. These plots can be compared with
Figure 7 and Figure 8 to show the effect of the presence of C1.
CH1 50.0mV CH2 50.0mV
M 50.0ms CH2 1.68mV
170.491kHz
2
1
CH1
p-p
74.0mV
CH1
NONE
CH2
p-p
34.0mV
CH2
MAX
16.0mV
CH2
MIN
–20.0mV
MEASURE
10803-012
Figure 12. Noise at Input (CH1) and Output (CH2) of HART Filter with 4 mA
Output Current, C1 Not in Place
CH1 50.0mV CH2 50.0mV M 50.0ms CH2 1.68mV
293.631kHz
2
1
CH1
p-p
132mV
CH1
NONE
CH2
p-p
44.0mV
CH2
MAX
20.0mV
CH2
MIN
–24.0mV
MEASURE
10803-013
Figure 13. Noise at Input (CH1) and Output (CH2) of HART Filter with 12 mA
Output Current, C1 Not in Place
Analog Rate of Change Measurements—AD5422 TSSOP
In terms of the analog rate of change test, the maximum peak result
with and without C1 in place were similar. The main difference
seen between the results was that without C1, the peak to peak
noise floor was much larger. Figure 14 and Figure 15 show the
analog rate of change plots for a slew rate of 120 ms (SR clock = 3
and SR step = 2) and 240 ms (SR clock = 5 and SR step = 2),
respectively.
CH1 50.0mV CH2 50.0mV M 50.0ms CH1 6.20V
<10Hz
2
1
CH1
p-p
8.00V
CH1
FREQ
4.371Hz?
CH2
p-p
198mV
CH2
MAX
104mV
CH2
MIN
–94.0mV
MEASURE
10803-014
Figure 14. AD5422 Output (CH1) and HART Filter Output (CH2), SR Clock = 3,
SR Step = 2, C1 = NC, C2 = NC
CH1 5.00mV CH2 50.0mV M 50.0ms CH2 6.20V
<10Hz
2
1
CH1
p-p
8.00V
CH1
FREQ
?
CH2
p-p
126mV
CH2
MAX
56.0mV
CH2
MIN
–70.0mV
MEASURE
10803-015
Figure 15. AD5422 Output (CH1) and HART Filter Output (CH2), SR Clock = 5,
SR Step = 2, C1 = NC, C2 = NC
Again, these plots can be compared with Figure 9 and Figure 10
to show the effect of the presence of C1. While the HART-coupling
technique used in this circuit configuration requires the use of
the external R
SET
resistor, note that even if the HART portion of
this circuit is not implemented, the addition of the buffer causes
a marginal degradation on I
OUT
accuracy when the internal R
SET
resistor is used. It is, therefore, recommended to use the external
R
SET
resistor when using this buffer configuration to tie the
voltage and current output pins together.
Rev. A | Page 9 of 10
