### DC AMPLIFIER MODEL A29
#
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 700 ohms, 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.
**DC AMPLIFIER MODEL A29 SPECIFICATION**
**Click here for Graph : Noise Characteristics**
**
**####
Noise
Equivalent noise resistance
approximately 700 ohms. 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 10 ohms 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 1 Megohm potentiometer across the internal supply, pins
11 and 12, and connecting a 100 Mohm 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 50K ohms.
Between 1K ohms and 50K ohms 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 1000
ohms, the noise voltage is constant down to zero ohms. 700 ohms is the amplifier
equivalent noise resistance (enr).
**
**##### Input Resistance
Greater than 1 Gohm, 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 2 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 10 ohms +/-
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) - 10 ohms.
Gain required = 1000, Rfb = 9.99K
ohms.
**
**##### 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 |