Welch Allyn Atlas Service Manual page 90

U504
3
To RL
S1
1
Sw-RLD-RL
IN1
14
To LL
S2
16
Sw-RLD-LL
IN2
11
To LA
S3
9
Sw-RLD-LA
IN3
6
To RA
S4
8
Sw-RLD-RA
IN4
13
fVcc V+
4
fVee V-
5
GND
DG202CSE
E
2.5.7 High Pass and Gain Stage
The signal from the output of the differential amplifier is AC coupled before a final stage of amplification.
The High Pass Pole is set to less than 0.05Hz (a 3.3 second time constant). Large DC signals are
sometimes present, for example, following a Leads Off condition. In order to quickly restore the baseline,
a transistor switch (Q504,Q505) is used to change the AC pole time constant to 0.33 seconds. Op Amp
U508 provides the final amplification prior to the A/D converter.
High Pass (0.05Hz or 0.5Hz)
2-pole low pass
C515 R550
3.3uF 19.6K
ecg-hp
Diff-out
R548 R549
1Meg 110K
Q504-d
E
Q504 1
2N7002
Q504-s
1
Q505
2N7002
E
2.6 Temperature Amplifier
The temperature option of Atlas is designed to operate with YSI-400 series probes. This probe has a
negative temperature coefficient, R@30C = 1815Ohms, R@40C = 1200Ohms. The temperature amplifier
is configured as a non-inverting gain stage. The positive input to the amplifier is a 1.25V reference. The
output of the temperature amplifier is V = Ref*(1+Rfb/Rthermistor).
The A/D converter will digitize the An-Temp input, along with the reference voltage and ground. The
gain of the A/D can be calibrated from the reference voltage measurement, and any drifts in the reference
voltage or A/D gain are compensated.
Appendix A
RLD Mux
2
D1
15
D2
10
D3
R516
10K
7 RL-mux
D4
Gain Stage
Gain = 74
fVcc
U508-5 5
+
7
6
-
C516
.047uF U508B
AD712
E
fVee
U508-6
R553
100K
Q504-g
Sw-MonBw
RLD Amp
fVcc
U503A
MC34002
3
+
1
2
E
-
U503-2
TP551
R527 fVee C514
1K 0.1uF
C514-2
R528
TP552
51.1K
An-ECG
R551 C517
73.2K .01uF
R552
1K
E
10/14/99
R525
10K
Diff+
R526
10K
Diff-