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Model 70A, Modular High-Performance Instrumentation Amplifier


  • Gains from 0.01 to 5,000
  • 0.1% or 0.02% gain accuracy
  • 100 kHz bandwidth
  • 300 Volt common mode
  • 2 or 6-pole selectable low-pass filter
  • Voltage substitution calibration
  • Dual, 100 mA, floating outputs
  • Optional AC input coupling


Model 70A is a full featured, wideband, differential instrumentation amplifier that can be used for a variety of applications. Designed to condition, amplify and filter low level analog signals it is used for data acquisition, analog recording, line driving, data filtering and signal isolation.

The basic element is a very stable, low noise differential amplifier with wideband response. Input impedance is 50 Megohms, linearity is 0.01% and common mode rejection is 130 dB for up to 300 Volts. Bandwidth exceeds 100 kHz for gains from 1 to 1000.

Eleven switch selectable gains and a variable gain multiplier provide continuous coverage from 1 to 5,000. Gain accuracy is ±0.1% with ±0.02% available as an option. The 70A-5 has a two pole, switch selectable, low pass filter with Bessel response. The 70A-6 is the same, but with six pole Bessel response. Butterworth response is also available.

Model 70A provides two independently buffered ±10 Volt full scale outputs each with 100 milliampere drive current. Output impedance is less than 0.1 Ohm for driving long lines and capacitive loads. Output is short circuit protected and current is limited to safeguard external equipment.

Front panel switches select gain step, calibrated or variable gain and filter frequency. Multiturn screwdriver controls set variable gain and input and output zero. The input attenuator and AC/DC coupling options have front panel controls.

Each amplifier is floating and line powered providing channel to channel isolation. It is fully enclosed in a metal case and mechanically secured when installed in the enclosure. Ten amplifiers mount in a 5 1/4 inch high rack enclosure with a rear mounted connector panel. Mating connectors are supplied with the rack enclosure.

Input  · Amplifier  · Filter  · Calibration  · Mechanical  · Ordering Information 



2 wire plus shield.

AC Coupling

Option P: Switch selected DC or AC input coupling with cutoff, -3 dB, frequency of 2.5 Hz. Input impedance with AC selected is 200 K Ohms.

Input Attenuator

Option C: Switch selected input attenuation of 0.01 ±0.1% providing gains from 0.01 to 5,000. Input impedance with attenuator selected is 2 Megohms, frequency response is 50 kHz and common mode rejection is 70 dB.



Eleven steps: 1, 2, 5, 10, 20, 50, 100, 200, 500, 1,000 and 2,000.

Variable Gain

Screwdriver adjustable gain multiplier increases the gain to cover the range between steps. Switch selects variable or calibrated gain. RTO errors and noise are multiplied by variable gain setting.

Gain Stability

±0.01% for 6 months, ±0.002%/ºC for fixed gain steps.

Gain Accuracy

±0.1%. ±0.02% with Option P input attenuator.

Gain Linearity

±0.01 %

Input Impedance

50 Megohms shunted by 200 pF.

Source Impedance

1,000 Ohms in any unbalance. Input may be grounded or floating.

Input Protection

Will withstand differential input of ±50 Volts and common mode voltage of ±350 Volts.

Common Mode

130 dB at DC, 123 dB at 60 Hz for gain 1,000, and 1 K Ohm source unbalance. At lower gains common mode rejection is 63 dB plus gain in dB. CMR decreases at a rate not exceeding 6 dB/octave from value at 60 Hz.

CM Voltage

Common Mode ±300V DC or peak AC operating.


Front panel controls provide independent adjustment of input and output zero.

Zero Stability

±5 µV RTI, ±0.5 mV RTO. Temperature coefficient is ±0.4 µV/ºC RTI, ±0.15 mV/ºC RTO.

Source Current

±1 nA, ±0.5 nA/ºC.

Noise (10 Hz)

1 µV RTI plus 0.5 mV RTO, peak.

Noise (10 kHz)

3 µV RTI plus 0.3 mV RTO, RMS.

Noise (100 kHz)

7 µV RTI plus 0.6 mV RTO, RMS.


100 kHz (-3 dB) for all gains 1 to 1,000. Bandwidth is reduced in proportion to gain above 1,000.

Slew Rate

3.1 V/µS RTO, 1.5 V/µS RTI.

Setting Time

75 µS to within ±0.1% final value.

Overload Recovery

Recovers from 5 times full scale overload to within ±0.1% in less than 400 µS at gain 1000, and faster at lower gains.

Analog Output (Amplifier)

±10 Volts at 100 mA. Output current limited to 150 mA. No instability with capacitive loads to 1 µF. Less than 0.1 Ohm in series with 5 µH. Each output may be configured by jumper for wideband or filtered response.


Filter (70A-5)

Switch selectable, 12 dB/octave Bessel low pass with 3 dB bandwidth steps of 1 Hz, 10 Hz, 100 Hz, 1 kHz, 10 kHz and Wideband.

Filter (70A-6)

Switch selectable, 36 dB/octave Bessel low pass with 3 dB bandwidth steps of 1 Hz, 10 Hz, 100 Hz, 1 kHz, 10 kHz and Wideband.


Other filter characteristics and cut offs are available.


Voltage Substitution

Voltage Substitution Amplifier input is switched to a voltage calibration bus for application of externally supplied calibration signal. Activated individually for each channel by a control input. Connecting the control input to output common selects calibration.


Rack Enclosures

19-inch wide enclosures hold 10 amplifiers in 5 1/4 inches of panel height. Depth is 20 1/4 inches. Convection cooling is usually adequate for up to 20 channels and a fan assembly is available for larger installations.

Signal Connectors

Input: XLR-3-32 (mate XLR-3-11C).
Output 1: MS3102A-24-28S (mate MS3106A-24-28P).
Output 2 MS3102A-24-28SX (mate MS3106-24-28PX).
Remote Control: MS3102A-10SL-4P (mate 3106A-10SL-4S).
Calibration: XLR-4-32 (mate XLR-4-11C).
Mating connectors and power cord are supplied.


115 or 230 VAC, ±10%, 47 to 400 Hz, 7 Watts.

Operating Temperature

0 to +55 ºC

Storage Temperature

-25 to +71 ºC


Up to 95% without condensation.

Ordering Information 

Ordering Information

70A-5 - Instrumentation Amplifier with 2-Pole Bessel Filter.

70A-6 - Instrumentation Amplifier with 6-Pole Bessel Filter.

Option A - ±0.02% Gain Accuracy.

Option BU - Butterworth Filter Response.

Option C - Input Attenuator (not available with option P).

Option P - DC/AC Input Coupling (not available with option C).

R10ACV115 - 10-Channel Enclosure 115 VAC.

R10ACV230 - 10-Channel Enclosure 230 VAC.

FM3A - Fan Assembly, 115 VAC.


Model 70A
(PDF, 101KB)

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