Precision Instruments .... phew !!!

[Samir]

Precision instruments are almost always costlier than
normal instruments. Unless you are working in a highly sofisticated
research laboratory, normal instruments will appear quite satisfactory
for most of the practical work. For example, consider two current
sources A & B that can supply a maximum of 10mA current over a voltage
range of 0 to 10V. Current source A is more accurate (+/- 0.1%) than
current source B (+/- 1%). Connect 1K resistor across current source A
& read IR drop by using 8 bit ADC (Analog to Digital converter).
Similarly, connect another 1K resistor across current source B & read
IR drop by using another 8 bit ADC. Both the ADCs are of same
accuracy. Following observations can be obtained by this set-up >>>>>

Current Source A (accuracy = +/- 0.1%) :
set current --- Actual Current --- IR Drop --- ADC value
(mA) ------------- (mA) ----------- (mV) ------ (HEX)

0.2 ------------- 0.2002 ---------- 200.2 ---------- 05
1.8 ------------- 1.7098 --------- 1798.2 --------- 2E
2.6 ------------- 2.6013 --------- 2601.3 --------- 43
3.8 ------------- 3.7962 --------- 3796.2 --------- 61
5.2 ------------- 5.2052 --------- 5205.2 --------- 85
6.7 ------------- 6.6933 --------- 6693.3 --------- AB
7.8 ------------- 7.7922 --------- 7792.2 --------- C7
9.2 ------------- 9.2092 --------- 9209.2 --------- EC
10.0 ------------- 9.9990 --------- 9999.0 ------ (1)00

Current Source B (accuracy = +/- 1%) :
set current --- Actual Current --- IR Drop --- ADC value
(mA) ------------- (mA) ----------- (mV) ------ (HEX)

0.2 ------------- 0.2020 ---------- 202 ------------ 05
1.8 ------------- 1.7820 --------- 1782 ----------- 2E
2.6 ------------- 2.6260 --------- 2626 ----------- 43
3.8 ------------- 3.7620 --------- 3762 ----------- 61
5.2 ------------- 5.2520 --------- 5252 ----------- 85
6.7 ------------- 6.7670 --------- 6767 ----------- AB
7.8 ------------- 7.7220 --------- 7722 ----------- C7
9.2 ------------- 9.2920 --------- 9292 ----------- EC
10.0 ------------- 9.9000 --------- 9900 -------- (1)00

16 Channel Analyzer Report for Current Source A
One point each in channel number 1,3,5,7,9,11,13,15 & 16

16 Channel Analyzer Report for Current Source B
One point each in channel number 1,3,5,7,9,11,13,15 & 16

Final report is the SAME for both accurate & inaccurate devices !!!