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410 & 420 Lock-in Amplifier - Over Drive for ±10V Output

Overview

 A number of customers have required a +/-10V output from the Scitec Instruments 410 and 420 lockin amplifiers. This application note gives details of how this can be achieved.

Standard Operation

 During standard operation the 410 and 420 units will produce output voltages and meter readings as specified in the following table:

Standard Operation (60dB Dynamic Reserve)

Gain Setting

Input Signal

Maximum Input Noise Signal Before Overload occurs

Output Voltage (*)

Meter Reading (*)

1V

1V AC RMS

14V Peak

+1V DC

Full Scale (+1)

300mV

300mV AC RMS

14V Peak

100mV

100mV AC RMS

14V Peak

30mV

30mV AC RMS

14V Peak

10mV

10mV AC RMS

14V Peak

3mV

3mV AC RMS

4.2V Peak

1mV

1mV AC RMS

1.4V Peak

300µV

300µV AC RMS

420mV Peak

100µV

100µV AC RMS

140mV Peak

30µV

30µV AC RMS

42mV Peak

10µV

10µV AC RMS

14mV Peak

3µV

3µV AC RMS

4.2mV Peak

* Assumes input signal is in phase with reference.

 An input signal of the same size as the gain setting will give full scale or 1V output. Signals smaller than the gain setting will give proportionately smaller outputs.

 When operated as above the unit has a dynamic reserve of 60dB.

Overdriving the Input to Give +/-10V Output

 A 10V output signal can be produced by reducing the gain setting by a factor of 10. No modifications to the 410 or 420 are required to do this.

Overdriving the input by a factor of 10 (40dB Dynamic Reserve)

Gain Setting

Input Signal

Maximum Input Noise Signal Before Overload occurs

Output Voltage (*)

Meter Reading (*)

1V

10V AC RMS

14V Peak

+10V DC

OVERLOAD

300mV

3V AC RMS

14V Peak

100mV

1V AC RMS

14V Peak

30mV

300mV AC RMS

14V Peak

10mV

100mV AC RMS

14V Peak

3mV

30mV AC RMS

4.2V Peak

1mV

10mV AC RMS

1.4V Peak

300µV

3mV AC RMS

420mV Peak

100µV

1mV AC RMS

140mV Peak

30µV

300µV AC RMS

42mV Peak

10µV

100µV AC RMS

14mV Peak

3µV

30µV AC RMS

4.2mV Peak

* Assumes input signal is in phase with reference.

 This method of operation has a number of disadvantages which need to be noted:

 The dynamic reserve of the system is reduced to 40dB.

 The meter reading is off scale. This will not harm the meter as the 410 and 420 instruments have meter protection circuitry built into them.

Factory Modification to Give +/-10V Output

 The internal circuity of the 410 or the 420 can be modified at the factory to give a +/- 10V Output.

Factory Modified +/-10V Operation (60dB Dynamic Reserve)

Gain Setting

Input Signal

Maximum Input Noise Signal Before Overload occurs

Output Voltage (*)

Meter Reading (*)

1V

1V AC RMS

14V Peak

+10V DC

Full Scale (+10)

300mV

300mV AC RMS

14V Peak

100mV

100mV AC RMS

14V Peak

30mV

30mV AC RMS

14V Peak

10mV

10mV AC RMS

14V Peak

3mV

3mV AC RMS

4.2V Peak

1mV

1mV AC RMS

1.4V Peak

300µV

300µV AC RMS

420mV Peak

100µV

100µV AC RMS

140mV Peak

30µV

30µV AC RMS

42mV Peak

10µV

10µV AC RMS

14mV Peak

3µV

3µV AC RMS

4.2mV Peak

* Assumes input signal is in phase with reference.

 There is no charge for making this modification if specified at any time before delivery.

 Care must be taken with these units that the time constant used on the output is sufficiently large to keep the output signal below about 12V DC. If the output has an average level of 10V but has a noise level of say +/- 5V then the signal above 12V DC will be removed causing the average output level to be lowered. This will cause the meter to read low.

 If you currently own a 410 or 420 instrument and wish to modify the unit yourself then please contact Scitec for details.

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