D.C. PICOVOLTMETER MODEL P12

The EM DC Picovoltmeter Model P12 is an ultra sensitive instrument which can amplify and measure DC potential down to a few picovolts (10e-12 volts) It is ideal for use as a null detector for very low impedance bridge balancing.
The noise performance is such that, using a source impedance of 30 milliohms or higher, the detection level will be limited by the Johnson noise of the source. The equivalent noise resistance of the P12 is only 30 milliohms. To put this figure into perspective, this is the noise generated by 30cm of 0.5mm diameter copper wire!     To make full use of the noise performance, it is desirable to use it with very low source impedance. While the P12 can be operated with source impedance of ten ohms or more, its characteristic will be overwhelmed by the source noise if the source impedance is too high. Great care must be taken with the input leads and connections to keep their resistance below the enr of the instrument, if possible.
In order to realise ultra low detection levels, the drift characteristics with temperature and time have to be maintained to levels which have been hitherto unattainable. the design of the P12 ensures that, even in normal laboratory environments, the lower ranges are easy to use.
The excellent stability of this instrument means that it is ideal for long term drift measurements, and may be coupled to a recorder and left unattended for long periods.
A six position filter gives the user control over the response time and reduction of noise from the circuit being measured.
A calibrated offset voltage control is provided, with an eleven position multiplier switch, giving a maximum offset of plus and minus 10 microvolts, which can be used to measure small changes in comparatively large signals.                                          The level of input which can be offset on any range will be limited only by the resolution of the calibrated potentiometer, i.e. about 0.01%.
The pure copper input terminal arrangement, with shield, allows easy connection to the circuit to be measured. A second input is provided, in conjunction with a switch on the front panel, which decreases the sensitivity of the basic ranges by one million. There are two outputs at the rear of the instrument, one direct and the other isolated. The direct output is used where maximum linearity is required although the isolator also has good linearity.
The instrument is battery operated from internal chargeable batteries and so has the advantage of complete isolation. There are two modes of battery charging. The instrument may be connected to the mains supply and use the specially designed multiple screened toroidal mains transformer, or a separate connector is provided so that an external current generator may be used to maintain the battery charge.  The latter mode may be useful when long term resolution of picovolts is required.
The P12 is engineered to a very high standard and comes complete with a terminal shield, which simply plugs onto the two outer front terminals.

SPECIFICATION D. C. PICOVOLTMETER MODEL P12

Click here for graphs : Noise and Impedance Characteristics

Range
12 basic ranges.
Input 1. +/- 100pV fsd to +/-30uV fsd.
Input 2. +/ - 100uV fsd to +/- 30V fsd. via the X10e-6 Attenuator

Accuracy
Direct Output +/- 0.5% at 23 deg C
Isolated Output +/- 1% at 23 deg C.
Meter +/- 2% fsd.
Attenuator +/- 0.5%

Zero Drift
Better than 100pV per degree C.

Noise
Equivalent noise resistance 30 milliohms. Peak to peak noise voltage depends on filter setting, e.g. filter time constant 30 seconds gives peak to peak noise voltage of approximately 8 picovolts with short circuit input.

Source Resistance
Up to 10 ohms. The full potential of the noise performance of the P12 will be realised at low source impedance, less than or comparable with the enr.

Resolution
Better than 5 picovolts, dependent on response time.

Input Impedance
Input 1. Approximately 30K ohms, 10 seconds after a step change in the input increasing with time to more than 10 M ohm.
Input 2. 10 M ohm.

Offset Current
Adjustable to zero at the front panel. Temperature coefficient better than 50pA per degree C.

Response Time
Depends on setting of filter and range Minimum 250 milliseconds. Maximum 300 seconds.

Filter
Six position low pass filter operated by a switch on the front panel.

Offset
10 turn potentiometer with dial, calibrated 0-100 on The front panel, in conjunction with an eleven position switch giving maximum offsets of plus and minus 1nV, 10nV, 100nV, 1uV, 10uV, and a '0' position. Resolution better than 0.01%. Accuracy better than 1%.

Series Mode Rejection
Greater than 80dB at 50Hz.

Common Mode Rejection
Greater than 180dB at 50Hz (Output to Input)

Outputs
Direct and isolated outputs giving 0 to +/- 1V for full scale deflection. Amplifier gain equals the reciprocal of the range setting in volts.

Isolation
Input to case. 100 G ohm. Input to Output. 100G ohm shunted by 1pF unbalanced capacity.

Connectors
Inputs. Low thermal emf pure copper terminals for basic ranges (Input 1) with plug on terminal cover. 'Input 2' and 'Case'. 4mm terminal socket. Outputs. 4mm sockets at rear panel.

Meter
Taut band movement type. 100mm Mirror Scale calibrated 10-0-10 and 3-0-3.

Power Supply
Internal rechargeable batteries giving more than 50 hours operation from a full charge. Internal mains powered charger fully charges the batteries in 16 hours with The instrument switched off or 20 hours when in use. Power required 110V or 240V 50Hz or 60Hz 5VA. There is an alternative input to the batteries to charge from an external current source. Minimum voltage 20V Battery capacity 1.2Ah.  NB. External charger must be a current source.

Dimensions
Width 314 (12.4")
Depth 322 (12.7")
Height 179 (7") Including folded feet.

Weight
28 lb. (12.5Kg).