5-2 Delta, Pulse Delta, and Differential Conductance Model 6220/6221 User’s Manual
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Operation overview
The Model 6220 or 6221 Current Source can be used with a Model 2182/2182A
Nanovoltmeter to perform Delta and Differential Conductance. The
Model 6221/2182A combination can also perform Pulse Delta. These operations
use a delta current-reversal technique to cancel the effects of thermal EMFs.
The Model 622x provides a bipolar output current and the Model 2182/2182A per-
forms A/D conversions (measurements) at source high and source low points. An
averaging algorithm is then used to calculate the delta reading.
Delta – The Model 622x provides a square wave current output, and the Model
2182/2182A performs A/D conversions (measurements) at each high and low out-
put level. A 3-point moving-average algorithm is used to calculate Delta readings.
As shown in Figure 5-1A, the first three Model 2182/2182A A/D conversions
(measurements) yield the first Delta reading. Each subsequent Model 2182/
2182A A/D conversion then yields a single Delta reading. Every Delta reading
uses the three previous A/Ds to calculate Delta.
Pulse Delta – The Model 6221 outputs pulses and uses 3-point repeating-
average measurements to calculate Pulse Delta voltage. For each pulse, the
Model 2182A performs an A/D conversion (measurement) at pulse low, pulse
high, and pulse low. Each set of three A/D readings yield a single Pulse Delta
reading. Figure 5-1B shows Pulse Delta measurements. If device heating is a
concern, 2-point measurements can instead be used (2nd low pulse not
measured due to corruption from heat).
Differential Conductance – The Model 622x outputs a differential current (dI)
sweep and measures differential voltage (dV). This function uses a 3-point
moving average algorithm to calculate dV. With dI known and dV calculated, the
Model 622x can then calculate differential conductance (dG) or differential resis-
tance (dR). Figure 5-1C shows Differential Conductance measurements.
NOTE Jitter – For Delta and Differential Conductance, step-to-
step timing may jitter as much as 1ms. This jitter can be
eliminated by disabling the front panel. For details, see
“Step-to-step timing jitter” on page 1-12.
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