Theory of Operation
Block Diagram Description2
Analog-to-digital conversion is accomplished within U1 using a modified dual-slope A/D
converter circuit, as shown in Figure 2-2. The conversion method in the Fluke 27 can be
described as a charge-coupled, multiple-slope technique. A series of 10 minor
conversions occur every 40 ms (each at 1/10th the desired resolution) without taking time
for an autozero phase between the conversions. These minor conversions (or samples, as
they are called in the following discussion) occur at a rate of 25 per second, and are used
to provided the fast response bar-graph display and fast autoranging.
New samples are taken every 40 ms. Ten samples are summed to produce a full-
resolution digital display, with full scale greater than 3200 counts. A 100 ms autozero
phase occurs every 10-sample sequence.
Basic A/D conversion elements and waveforms are illustrated in Figure 2-2. As this
figure shows, a residual charge is retained by the integrator capacitor due to the overshoot
past the true-zero base line. In the absence of an autozero phase, the residual charge
would normally produce a significant error in the sample next taken. However, a digital
algorithm eliminates the error and accounts for the residue as it propagates through all 10
samples.
aac02s.tif
Figure 2-2. A/D Conversion Elements and Waveform
2-5 |
ACEC
OTHERS
