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Analog Buffer and Inverter Switching with logic - del20016


Here the 4053 selects or routes the voltage, current or resistance measurements to the A-D converter or display. It is selected with the mode selection when you want to measure Volts, Amps, Ohms and AC-DC. Some have to be polarity inverted and some signals just buffered this is selected and done by this circuit according to the digital control. 

Now U1 OP07 circuit is a Buffer unity gain and low offset, U2 circuit is unity gain but polarity of output is opposite of input. D1-D2-R3 form a AND gate to select diode-buzzer test mode. The digital selection of 4053 Analog-Switch does not produce any errors in the analog-switching of even mV signals. But it works best at +/- 7.5V dual supply pin-16 is +7.5V, pin- 8 is digital ground and pin 7 alone should go to -7.5. Analog ground can be same as digital ground, or the switched signals must be within +/-5V of digital ground. The switches should not carry any current and should be buffered at the output by FET opamps 1-Tera-Ohm. Then alone measurements are ok, as the switches have ohmic resistance.

Analog
                        Buffer and Inverter Switching with logic
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Analog Buffer and Inverter Switching with logic - del20016

OP-07 from Analog Devices is a Ultra-low Offset Voltage Operational Amplifier. It has a reasonable input impedance of around 75uV and 1 gigaohm.

OpAmps for DC, Audio, and Near-RF use have to be carefully selected. You need not look at ideal levels for all parameters for all types. What is needed for AC low signal may not be needed in DC Bridge measurements. An ideal OpAmp in all respects is a luxury we do not need. But in some research applications, even that is required. A Wide bandwidth in combination of Low Offset and ultra-low bias, looks interesting for R&D. The DMM modes table is below.

Mode
B
A
Volts
0
0
Kilo Ohms
0
1
Milli Amps
1
0
Diode Test
1
1

Data sheets and info useful to study and build this circuit.