In the third part of this series, Build Your Own Curve Tracer, Part 3: TPA202 Output Port Circuit, the TPA202 output-port circuit was described. The two port SMUs are the main functions of the TPA, though a display of values and a supply are necessary auxiliary functions, described in this Part 4 which continues the summarized circuit description derived from the TPA202 Manual.
The TPA202 displays voltage and current values on a DVM display. The DVM circuit is shown below.
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The DVM is a modified dual-slope A/D converter with LED 3.3 digit + sign display. The input has a bipolar range of ±1 V. The functional diagram is shown below. N = 3; IR = 200 μA; RX = 20.0 k
; C = 4.7
F; IX(fs) = –50 μA.
The semi-discrete DVM could be implemented with a 40-pin legacy DVM IC. These ICs have 4.3-digit capability and are comparable in cost to the semi-discrete design. The semi-discrete alternative was chosen to provide more circuit observability, design versatility, a zero-scale feature, achieve design re-use, provide ADC tutorial value, and invoke some nostalgia. A slightly superior alternative at 3.3-digit resolution is the optimal integrating ADC, a
converter. However, it requires almost as many parts for a DVM. This DVM design has been refined to avoid typical zero-scale problems with dual-slope ADCs that
converters inherently avoid, and is thus only slightly suboptimal.
The design challenge is to minimize parts while maintaining performance. The 3-digit LED driver uses only 2 ICs plus a dual flop for leading “1” and overflow indication of an invalid reading. This DVM design, in slightly modified form, is also used in the Innovatia Floating Differential Source (FDS) and in the TPA204, the FDS-to-TPA adaptation, which is a computerized version of the TPA202. In the TPA204, the analog part of the modified-dual-slope DVM is retained, and the DVM becomes a 14-bit ADC with counter and display driver in an Atmel ATmega8515