Crystal Oscillator – Parallel Resonant

74HCU04 is a chip that was made for this purpose, HCT may not work for such a circuit. C1 and C2 can go to upto 33pF and R2 can be increased to make R2 * C2 = t.

Crystal Oscillator - Parallel Resonant

 

Time constant much less than the period T of the crystal T = 1/F . This is to remove higher frequency components in the Oscillator.

More on Piezoelectric Crystal Oscillator

The circuit above is a parallel resonant oscillator circuit. The Crystal works by the piezoelectric principle, piezo means pressure. The electric field causes the impedance of the crystal to change. The LP Record Player needle is the reverse of this, the bumps on the spiral groove of the record applies pressure to needle which generates electricity. Both are piezo-electric effects.

Precision Attenuator for Digital uC Control

When Instruments are designed a analog front end is essential and also as most equipment have digital or microcontroller interface the analog circuit needs to have digital access. The Circuits DACT0008 and DACT0009 are both useful in building instruments which have digital control.

Precision Attenuator with Digital Control

The Circuit DACT0008 is a programmable attenuator and the digital control can be a remote dip switch, a CMOS Logic Output like the A-B-C-D outputs of a decade counter, or an I/O port of a uC like 80C31.

The heart of the circuit is the popular OP07 OpAmp with Ultra Low Offset in the inverting configuration, 4052 a CMOS analog multiplexer switch enables the gain change, the innovation of the circuit is that the on resistance ( around 100 ohms) of 4052 switch is bypassed so that no error is introduced by its use.

The resistors used R1 to R6 can be 0.1% 50ppm if you will use a 3 ½ DPM i.e. + /- 1999 counts ( approx. 11 bit ), but for 4 ½ DPM ( approx. 14 bit ) you may need to have trimpots2 in place of R3, R4, R5 & R6 gain selection resistors to properly calibrate to required accuracy but for testing or trials use 1% 100ppm MFR resistors but the errors will be around 1%.

Precision Attenuator with digital control

b. Output

Output connect to DPM 7107/7135 or any other A/D Convertor or OpAmp Stage. Use a buffer at output if output has to be loaded by a value less than 1Meg. Use an inverting buffer if input leads have to have polarity where gnd is -In. See DACT0009 for details.

c. 4052 CMOS Switch

The 4052/51/53 Analog Multiplexers have an on Resistance of around 100E the highlight of the circuit is that the CMOS on resistance comes in series with the opamp output source resistance, which produces no error at output.

Digital Control Options

A and B can be controlled by I/O port of uC, like 80C31 so that the uC can Control gain. A and B can be given to Counters like 4029/4518 to scroll gain digitally. A and B can be connected to DIP switch or thumbwheel switch.

Precision Amplifier with Digital uC Control

When Instruments are designed a analog front end is essential and also as most equipment have digital or microcontroller interface the analog circuit needs to have digital access. The Circuits DACT0008 and DACT0009 are both useful in building instruments which have digital control.
This circuit DACT0009 is similar to DACT0008 but gains of upto 100 can be realized in this configuration, this is useful for signal conditioning of low mV outputs of transducers. The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1K to 1M and can also be trimpots for obtaining gains as required by user, the resistor values shown are for decade gains e.g. for an auto ranging DPM.

Precision Amplifier with Digital Control

R1 and C1 reduce ripple in input and also snubs transients, ZD1 and ZD2 Zeners clamp input to +/- 4.7V the input current is limited by R1 lastly C1 and C2 are decoupling capacitors. The OpAmp U3 is used to increase the input impedance so that very low mV inputs are not loaded on measurement, the user can terminate the inputs with a resistor of his choice like 10M or 1M to avoid floating of the inputs when no measurement is being made. U5 is used as an Inverting buffer to restore polarity of the input and U4 is used as a buffer on the output of 4052 because loading it by resistance of value less than 1M will cause an error. An alternative is use R7 = R8 =1M and remove U4 but this may not be ideal. Gains of greeter than 100 may not be practical because at 100 gain itself a 100uV offset will be around 10mV at the output (100uV*100) this can be trimmed using the offset null option in the OP07, connect a trimpot between 1 and 8 and connect wiper to +5.

Precision Amplifier with Digital Control

For better performance use ICL7650 ( not pin compatible ) instead of OP07 and use +/- 7.5V instead of +/-5V supply.

Eight steps for gain or attenuation can be added by using two 4051 and by using Pin 6 Inhibit on 4051/52 limitless steps can be added by cascading many 4051,52,53 as Pin 6 works like a chip select.

Some extended applications of this circuits are……. Error correction in Transducer amplifiers by correcting gain. Auto ranging in DMM. Sensor selection or Input type selection in Process control. Digitally Preset power supplies or electronic loads. Programmable Precision mV or mA sources. PC or uC or uP based instruments. Data loggers and Scanners.

Digital gain control of Opamp.

The gain of U1 can be controlled by a digital binary 1248 nibble at ABC. The gain at digital 000 is unity or 1 and the gain at various stages are set by 4051.

Precision Attenuator with Digital Control – delabs

There are eight different gains as the steps of gain resistor network is chosen by 4051. The on resistance of 4051 channel around 100E gets added to U1 pin 2 internal impedance.

Digital gain control of Opamp

Auto ranging 4-1/2 Digit Digital Voltmeter – delabs

You can use separate resistor networks with trimpots for each channel if you require but keep the networks total burden on U1 pin 6 to around 10K, not less than than. You can use this to set the gain of a amplifier with the help of a microcontroller.