DC Amplifier Model A29

Description

The EM DC amplifier model A29 is a low noise amplifier module, for sensitive DC measurements, data collection and systems, and is ideal for sensitive temperature measurement using thermocouples. The noise level of the A29 is equivalent to a perfect resistor of about 700Ω, thus giving sensitivity of 7 nanovolts with a response time constant of a second. When used with normal type thermocouples for temperature measurement, sensitivities of a few hundred micro Kelvin can be achieved. The input voltage drift is very low and is compatible with voltage sensitivities of a few nanovolts. The A29 has many other features desirable in a measurement amplifier. The very high loop voltage gain of about 1T, or 240dB, means that high overall gain may be used, controlled precisely by feedback resistors, thus ensuring good linearity, with the accuracy defined by the feedback resistors.

Despite the high gain, the model A29 is stable with 100% feedback, thus allowing feedback capacitors to be used to filter the output. The input signal level can be up to plus or minus 20 milli-volts and the output can be up to plus and minus 6 volts.

The response time of the A29 is fast, and the loop gain is reduced to unity at about 300 kHz, thus ensuring good response times, even at high gain settings.

The power supply requirement is low, being about 3 milli-amps with power supplies of plus and minus 12 volts, making it ideal for multi-channel measurement systems.

The A29 is designed to be mounted on a printed circuit board, using a 0.1″ grid, with 13 pin connections on a dual in line rectangle 1.6″ X 1.2″, and is built into a stainless steel case which gives it immunity from interference both electrical and thermal.

 

Specification: DC Nanovolt Amplifier Model A29

Noise

Equivalent noise resistance approximately 700Ω. Noise voltage depends on bandwidth selected, e.g. rise time constant 1 second, peak to peak noise voltage is approximately 7 nanovolts (for 99% of the time) with the rms equivalent being about 1.4 nanovolts.

Offset Voltage

This is adjustable to zero by introducing a current into the feedback terminal. The internal feedback resistor is 10Ω and so an offset current of plus and minus 100 nano-amps will give a voltage offset in the range of plus and minus 1 microvolt. This is easily achieved by connecting a 1MΩ potentiometer across the internal supply, pins 11 and 12, and connecting a 100MΩ resistor from the wiper to the feedback connection, pin 4.

The temperature coefficient of offset voltage, DC, is typically 5 nV per degree C. There is also a differential temperature coefficient, up to 10 nV per degree C. depending on the rate of change of ambient temperature. This is negligable for normal environments, with a low rate of change. The initial, unbalanced offset voltage is typically plus or minus 500nV.

Offset Current

The initial, unbalanced, offset current is typically 50pA.

The temperature coefficient of offset current is typically 2 pA per degree

Source Resistance

The A29 closely follows the Johnson noise characteristic from its equivalent noise resistance up to about 50KΩ. Between 1KΩ and 50KΩ source resistance, the total noise is very little more than the noise from the source alone, i.e. the noise increases by about 10dB per decade. At source resistance above about 50K, the overall noise increases above the source noise by about 10dB per decade, giving an overall increase of noise of about 20dB per decade. At source resistance below 1000Ω, the noise voltage is constant down to zero ohms. 700Ω is the amplifier equivalent noise resistance (enr).

Input Resistance

Greater than 1GΩ, DC

Gain and Bandwidth

Open loop voltage gain 1T or 240dB. This gain is reduced to unity at 300KHz, at a rate of 32dB per decade. The amplifier is stable with 100% feedback; the limitation to this is the current that can be taken from the output stage, which flows through the 10

ohm feedback resistor.

Input Level

The A29 will operate with input levels up to plus or minus 20 mV.

Output Level

The A29 will provide an output level up to +/- 6 volts at DC and will deliver +/- 2 mA.

The output is limited at low gain settings by the maximum output current which can be drawn from the amplifier output. Thus, if the gain is set to 100, the maximum output level is 200mV.

Feedback Components

There is an internal feedback resistor connected to the Feedback terminal and internal ground, of 10Ω +/- 0.1%, with a t.c. of 25ppm/degree C.

The gain is set by connecting an external resistor from `Output’ to `Feedback’.

The value of this resistor is:

Rfb = (gain required X 10) – 10Ω.

Gain required = 1000, Rfb = 9.99KΩ.

Connections

All connections to the A29 are made by printed circuit board pins, which are configured on a 0.1″ grid, on a rectangle 1.6″ X 1.2″. The pcb mounting hole should be 1 mm diameter.

Power Supply

The A29 operates from supplies of +/- 11 volts to +/- 15 volts. The nominal operating supply should be +/- 12 volts. Current required is +/- 3mA.

Dimensions

Length 2.5″ (61mm)

Width 1.73″ (44mm)

Height above board 0.69″ (17.5mm)

Printed Circuit Board Pin Length 0.22″ (5.5mm)

Connection Pin Size 0.025″ (0.64mm) square.

Weight

3.3 ounces (88 gm)

Note

A utility pc board is available for mounting the A29, which has a passive filter, which may be by-passed and the following facilities.

Voltage offset control, powered from the internal regulated supplies.

Pins for input, direct output, filtered output, power supply, feedback and filter connections.

Board Size 3.5″ X 3″

This specification may change without notice

X = 10 Seconds per Division
Y = 5 Nanovolts per Division
Measurement Rise Time Constant = 1 Second