When instruments are designed, an analogue front end is es- sential. Further, as most equipment have digital or microcontroller interface, the analog circuit needs to have digital control/access. The circuit described here is that of a programmable attenuator with digital control - which can be a remote dip switch, or CMOS logic outputs of a decade counter (having binary equivalent weight of 1, 2, 4 and 8 respectively) or an I/O port of a micro-controller like 80C31.The heart of the circuit is the popular OP07 opamp with ultra low offset, in the inverting configuration. A dual, 4-channel CMOS analog multiplexer switch CD4052 enables the gain change. The innovative feature of the circuit is that the on resistance (around 100-ohms) of CD4052 switch is bypassed so that no error is introduced by its use.

The resistors R1 to R6 used in the circuit should be of 0.1 percent tolerance, 50 ppm (parts per million) if you use it with a 3 ½ digit DPM i.e. ± 1999 counts (approx. 11 bit), but for 4 ½ digit DPM (approx. 14 bit) you may need to have trimpots (e.g replace resistor R6, 1 kilo-ohm by a fixed 900-ohm resistor in series with a 200-ohm trimpot) to replace R3, R4, R5 and R6, gain selection resistors for proper calibration to required accuracy. However for testing or trials, use 1 per cent 100ppm MFR resistors. The expected errors will bearound 1 per cent.

To keep parts count (hence cost) to a minimum, the common or ground is used as the positive input terminal and the negative being one end of resistor R1. This is so, because the opamp inverts the polarity as it is used in inverting configuration, this does not matter as the equipment will be isolated by the power supply transformer and all polarities are relative. In case you want the common to be the negative, then you will have to add the stages (IC4 and IC5 circuitry shown in precision amplifier circuit described later).

The OP07 pin out is based on standard single opamp 741 and any other opamp like CA3140, TLO71 or LF351. The later opamps can be used but with a lot greater offset errors- in excess of 1 per cent and this is not tolerable in precision instrumentation.

The OP07 has also equivalents like uA714 and LM607 having ultra low offset (voltage) < 100uV and low input bias (current) <10nA and high input impedance >100M are the key requirements for a good instrumentation opamp for use with DC inputs.

Design considerations. (a). Input: 500V max. Note that ¼ W resistors can withstand upto 250V hence resistors R1 and R2 in series are used for 1-Meg-ohm with 500V (max) input limit. These resistors additionally limit the input current as well. Diodes D1 and D2 clamp the voltage across input of opamp to ±0.5V, thereby protecting the opamp.

(b). Output Output can be connected to a 7107/7135-based DPM or any other analogue to digital converter or opamp stage. Use a buffer at output if output has to be loaded by a load less than 1 Meg-ohm. Use an inverting buffer if input leads have to have polarity where ground is the inverting terminal. (for details see next circuit) C.

(c) CD4052 CMOS switch The on-resistance (100-ohm approx.) comes in series with the opamp output source resistance, which produces no error at output.

Caution: The circuit does not isolate, it only attenuates. When high voltage is present at its input, do not touch any part of the circuit.

(d) Digital control options
(i). A and B can be controlled by I/O port of µC like 80C31 so that the µC can control gain.
(ii) A and B can be given to counters like 4029/4518 to scroll gain digitally.
(iii) A and B can be connected to DIP switch.
(iv) A and B can be connected to a thumbwheel switch.

Notes.

1. Digital inputs logic 0 is 0V logic 1 is 5V.
2. All resistors are metal film resistors (MFR) 1% unless specified otherwise.
3. C2 and C3 are ceramic disk 104 = 0.1uF = 100n.