Instructions for Use
– SCHMIDT
®
Flow Sensor SS 20.700
Page 19
Use of only one analog output
It is recommended to connect the same resistance value to both analog
outputs, even if only one of them is used. For example, if only the an-
alog output
“flow velocity” is operated as current output with a load of
a few ohms, it is recommended to connect the other analog output
(“medium temperature”) with the same resistance value or directly to
AGND.
Nominal operation
The measuring resistance R
L
must be connected between the corre-
sponding signal output and the electronic reference potential of the
sensor (see Figure 4-3). Typically, AGND must be selected as meas-
uring reference potential for the signal output. The supply line GND can
also be used as reference potential, however, the ground offset can
cause si
gnificant measurement errors in the “Voltage” operating mode.
AGND should generally be selected as reference potential for
the analog signal outputs.
Figure 4-3
Depending on the load resistance R
L
, the signal electronics switches
automatically between its operation as voltage interface (mode:
“U”) or
current interface (mode:
“I”), hence the designation “Auto-U/I”.
The switching threshold is in the range between 500
… 550 Ω (for de-
tails refer to
chapter 5 Signaling
). However, in voltage mode a low re-
sistance value may cause significant voltage losses via the line re-
sistances R
W
of the connection cable which can lead to measuring er-
rors.
For voltage mode, a measuring resistance of at least 10 k
Ω is
recommended.
The maximum load capacitance C
L
is 10 nF.
