Current flow/splitting on the Edge guard and Bias ring
Measurements at UCSB, January 15/16, 1998
During the grounding and shielding tests in November/December at UCSB
we made the puzzling observation that the splitting of the
current in the Edge guard and Bias ring P was dependent on the
value of the series resistor of these lines on the Matching Card.
For a value of 1k, there was no current measured on the Edge guard,
but all current was going through the bias line. However, for 100k
series resistors, the currents on Edge guard and Bias line were
of about the same size. The total amount of current in both lines
was the same for 1k and 100k.
These measurements were done with the 3x3 test module at UCSB.
Last week we performed a set of measurements to investigate this
strange effect in more detail.
Setup
- The currents in the Bias ring and the Edge guard are
measured simultanously. Both lines are connected after
the ampmeters to the same power supply.
- The 4 resistors on all the bias lines
(2 x Bias ring N, Bias ring P,
Edge guard P) on the Matching Card are replaced with potentiometers
with a range of resistance between 0 and 200k.
- The measurements were performed with both available modules,
the 3x3 test module (V_bias=40V) and the H1 test module
(V_bias=35V).
Results
Measurements on the 3x3 test module
The following table list the measured current values for
the Edge guard (I_E) and Bias ring (I_B) for different
values of the series resistors on the Matching Card.
The impedance on the Bias Ring N lines was set to 100k as well
as 1k, and as expected, did not influence the current flows
(apart from a slight change in the total current, of course).
R_B
(kOhm) |
R_E
(kOhm) |
I_B
(uA) |
I_E
(uA) |
I_tot
(uA) |
I_E/I_tot |
| 0.92 |
0.94 |
3.20 |
0.00 |
3.20 |
0.0 |
1.0 |
1.0 |
4.10 |
0.05 |
4.15 |
0.01 |
| 2.0 |
2.0 |
3.80 |
0.22 |
4.02 |
0.05 |
| 5.0 |
5.0 |
3.45 |
0.54 |
3.99 |
0.14 |
| 10. |
10. |
2.81 |
0.80 |
3.61 |
0.22 |
| 20. |
20. |
2.25 |
1.01 |
3.26 |
0.31 |
| 35. |
35. |
2.30 |
1.35 |
3.65 |
0.37 |
| 50. |
50. |
2.13 |
1.42 |
3.55 |
0.40 |
| 70. |
70. |
1.95 |
1.46 |
3.41 |
0.43 |
| 100. |
100. |
1.77 |
1.43 |
3.20 |
0.45 |
| 183. |
183. |
1.74 |
1.54 |
3.28 |
0.47 |
| 20. |
1.0 |
1.82 |
1.50 |
3.32 |
0.45 |
As can be seen from the last row of the table, by changing the
impedance on one line individually, one can force the current
onto a different path.
Sum of Bias ring and Edge guard current as
a function of series impedance.
Ratio of Edge guard current over
total current as a function of series impedance.
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