Tektronix AM502 Diff. Amp 1-100K Gain, DC-1MHz BW

Back to Tek TM500 Series Summary Page

BAMA AM502 Free Manual Download Site
Bitsavers AM502 Free Manual Download Site

Quick Specifications / Photos
Detailed Specifications
Operating Instructions
Misc. Part Numbers

Samples of e-Bay Prices
Orig CostLow BidHigh Bidaverage
$675 (1977)
$1065 (1982)
$1260 (1985)
$75.00 7/23/01 #1617692418 ridgeeq@erols.com
$75.00 7/23/01 #1617706586 n1yob
$74.00 8/10/01 #1623705606 bbg
$52.00 8/17/01 #1626128970 bbg
$102.50 8/23/01 #1628120454 jean15
$100.00 8/23/01 #1628224764 lhw43
$82.03 8/24/01 #1628569472 bbg
$162.50 9/5/01 #1631800154 paulasparkwheel
$76.00 9/14/01 #1636245312 bbg
$99.00 9/20/01 #1638367007 jshank
$56.00 9/21/01 #1638519872 jds8
$44.01 9/24/01 #1639732051 reliableendmill
$61.00 9/29/01 #1642358680 televisionary
$58.77 10/5/01 #1645545574 bbg
$62.01 10/9/01 #1645554364 reliableendmill
$102.50 10/16/01 #1648441356 ridgeeq
$113.50 10/29/01 #1653571352 abeja
$104.01 11/5/01 #1656625545 rhinoenter
$100.00 3/1/02 #1707656051 lcoppes
$104.86 4/10/02 #1719952287 sales(astglobal)
$118.45 1/24/03 #3108739400 joepalaia (untested) (11 bids $40 start)
$59.00 2/4/06 #7585846808 andpower79 (8 bids $3.99 start) (worked when last used but untested)
$61.00 (+$15 ship) 12/23/06 #160064933480 electronicswest (10 bids $1 start) (testing not spec'd)
$119.50 (+$10-$12 ship) 1/6/07 #120070304656 1ettech (10 bids $0.01 start) (tested-clean)
$38.00 (+$12 ship) 3/23/07 #270101124539 ice6575 (1 bid) (untested)
$43.51 (+~$16 ship) 6/27/07 #290131190538 tekyard (6 bids $0.01 start) (untested)

  • DC to 1 MHz Maximum Bandwidth
  • Gain of 1 to 100,000
  • 100 dB Cmrr
  • Selectable Upper and Lower -3dB Points
  • Adjustable DC Offset
    Compatible with P6101A probe (per 1985 catalog)


    The AM 502 is a dc-coupled differential amplifier with excellent common-mode rejection capabilities and high gain for low voltage measurements. The dc offset capability permits nulling up to 1 volt dc, so that low level, low frequency signals impressed on a dc level can be amplified without the degradation often introduced by ac input coupling. High and low frequency -3dB points can be selected at the front panel to suit the application. Signal inputs and outputs are available at the rear connector as well as at the front panel. A front panel lamp indicates most overrange conditions of excessive input signal, excessive gain, or excessive offset.

    The input circuit can be represented by approximately 1 megohm ro ground paralleled by approximately 47 picofarads. The input impedance can be raised to about 200 megohms with the removal of a jumper.

    Overload protection is provided by fuses in series with the input which open when large amounts of current flow due to overloading conditions.

    A SETP GAIN DC BALANCE control is provided to adjust for zero shift as the gain switch is changed from one position to another.

    The DC OFFSET COARSE and FINE controls give offset up to ±1 volt dc potential difference at the input connectors. The amplifier internal bias is changed to accomplish the offset. The LF -3dB switch must be in the DC OFFSET position to actuate these controls.

    The HF -3dB switch is used to reduce the upper bandwidth limit as necessary to improve the signal-to-noise ratio when using the AM 502 in low-frequency applications. The LF -3dB control increases the lower bandwidth frequency.

    Use of the pre-charging feature prevents surge currents, due to charging the ac coupling capacitor, from damaging the circuit under test.


    CharacteristicsPerformance RequirementsSupplemental Information
      Normal Mode100 to 100,000 within 2%,
    10 steps in 1, 2, 5 sequence.
    ÷ 100 Mode1 to 1000 within 2% 
    Variable Range Continuously variable uncalibrated gain to at least 2.5 times the calibrated gain setting.
    Frequency Response
    Direct Coupleddc to 1 MHzWith Gain control set to 20K or less, selectable HF -3dB points are with +1dB, -2dB.
    Upper -3dB point reduces to 500 kHz at 50K gain, and 250 kHz at 100K gain.
    AC Coupled2 Hz or less, at lower frequency -3dB point to 1 MHz 
    HF -3dB100 Hz to 1 MHz, 9 steps in 1-3 sequence.Reference approximatelyh 5.0 V peak-to-peak output at 1 KHz.
    Accuracy+1 dB, -2 dBReading range -2 dB to -5 dB.
    LF -3dB0.1 Hz to 10 kHz, 6 steps in 1-10 sequence. 
    Accuracy+1dB, -2dBReading range -2dB to -5dB
    Voltage Swing±5 V. 
    Current ±20 mA
    Ro 5 Ω or less
    Minimum Load Impedance 250 Ω
    Common Mode
    Normal Mode
    ±5 V 
    Rejection Range
    100 dB, dc to 50 kHzDirect coupled with Inputs shorted together.
    ÷100 Mode
    ±50 V 
    Rejection Range
    50 dB, dc to 50 kHzDirect coupled with less than 100 V peak-peak sine-wave input.
    DC Offset
     At least + and - 1 V.
    Maximum Safe Input Voltage
    Direct Coupled
    Normal Mode
     15 V (dc plus peak ac) to 5 MHz or less.
    ÷ 100 Mode
     350 V (dc plus peak ac) to 5 MHz or less.
    AC Coupled 350 V (dc plus peak ac) with coupling capacitor precharged.
    Maximum Input Gate Current (each Input) 50 pA at 25°C.
    Noise (Tangentially Measured)25 μV or lessWith 25 Ω or less source resistance, from 5 Hz to 1 MHz
    Voltage Drift with Time
    Short Term 10 μV (peak-to-peak) per hour
    Long Term 20 μV (peak-to-peak) per hour
    Voltage Drift with Temperature 100 μV per °C
    Input R and C Resistance, 1 MΩ
    Capacitance, Approximately 47 pF

    above excerpts from 1977 TM 500 brochure and AM 502 DIFFERENTIAL AMPLIFIER Instruction Manual


    Input Connections

    When making single-ended input measurements (conventional ampifier operation), be sure to establish a common ground connection between the device under test and the AM 502. The shield of a coaxial cable is normally used for this purpose. Push the GND button for the input not connected to the device under test.

    In some cases differential measurements require no common chassis ground connection, and therefore are less susceptible to interference by ground-loop currents.

    Consider the change in the source operating characteristics due to loading by the signal input cables. The circuit at the input connectors can be represented by approximately 1 megohm to ground paralleled by approximately 47 picofarads. Two feet of 50 ohm coaxial cable increases the parallel capacitance by about 60 picofarads, which could be excessive in many situations. To minimize these effects, use a higher impedance cable or an attenuator probe.


    Attenuator probes decrease the resistive-capacitive loading of a signal sorce. They also extend the measurement range of the AM 502 to substantially higher voltages. Some measurements require a higher resistance input to the AM 502, with very little source loading or signal attenuation. In such situations use a FET probe or the high-impedance input provision of the AM 502.

    High Impedance Input

    To raise the internal input impedance of the AM 502 to about 200 megohms, remove the P40 plug.(on the circuit board near the AC/DC/GND switches). Make certain the attenuator is in the NORM mode. Signal source impedance now becomes an important factor. For example, a 100 picoampere gate current through 10 megohms produces a one-millivolt offset. This offset may result in significant error when small voltages are measured.

    Input Overloading

    When measuring unknown dc voltages, push the ÷100 pushbutton until a suitable output signal is obtained. If the input circuit of the AM 02 is overdriven, large amounts of current will flow, opening the protective fuses.

    Output Connections

    To prevent current limiting in the output stage, do not load the output with less then 250 ohms. Output current is limited to 20 milliamperes.

    Step Gain DC Balance

    If this control is misadjusted, the dc output level will shift as the GAIN switch position is changed. Push both GND buttons and place the GAIN switch in the 100 position. Rotate the GAIN switch from stop to stop while adjusting the STEP GAIN DC BAL control for no dc shift at the OUTPUT terminal.

    DC Offset Coarse and Fine

    Use these controls to offset up to ±1 volt dc potential difference at the input connectors. The amplifier internal bias is changed to accomplish the offset. The differential rejection capabilities of the AM 502 are not affected. The LF -3dB switch must be in the DC OFFSET position to activate these controls.

    HF and LF Bandwidth Reduction

    Use the HF -3dB switch to reduce the upper bandwidth limit, as necessary, to improve the signal-to-noise ratio when using the AM 502 in low-frequency applications. The LF -3dB control increases the lower bandwidth frequency. Use this control to reduce dc drift, when raising the lower bandwidth does not undesirably reduce the bandwidth for the signal being measured.


    Use of this feature prevents surge currents, due to charging the ac coupling capacitor in the AM 502, from damaging the circuit under test. Before connecting the AM 502 to a signal containing a dc component, push the AC and GND pushbuttons. Connect the input to the circuit under test. Wait about one second for the coupling capacitor to charge. Release the GND pushbutton, and the coupling capacitor is charged to the value of the dc voltage to be measured.

    Differential Operation

    A differential measurement is made by connecting each of the two inputs to selected points in the test circuit. The input to the amplifier will then be the difference in voltage of the two selected points. Consideration should be given to the proper connection method used between the AM 502 and the circuit under test, otherwise improper measurement results may occur. Figure 1-4 in the Instruction Manual shows the proper and improper connections.

    Differential voltage measurements are made by applying the signals to the + input and - input connectors. Set the input coupling switches to the same position, AC or DC, depending on the measurement being made. In differential measurements, only the voltage difference between the two signals is amplified. Common mode signals (common in amplitude, phase, and frequency) are rejected.

    Single-ended measurements often yield unsatisfactory results because of the interference resulting from ground-loop currents between the AM 502 and the device under test. In other cases, it may be desirable to eliminate a dc voltage by means other than the use of a blocking capacitor, which could limit the low-frequency response. The limitations of single-ended measurements are effectively eliminated by using differential measurements.

    DC Offset Operation

    By using the FINE and COARSE DC OFFSET controls, it is possible to use the AM 502 differentially in a slide-back mode, to observe a small signal whose dc potential difference may be considerable. The offset voltage is continuously adjustable from plus 1 volt to minus 1 volt., and is internally available for all of the GAIN switch positions. The LF -3dB switch must be in the DC OFFSET position to activate the offset circuit.

    Common Mode Rejetion Ratio

    The ability of the AM 502 to reject common-mode signals is indicated by the common-mode rejection ratio (cmrr). For example, assume that the signal consists of unwanted 60 hertz at 10 volts peak-to-peak (common mode connected to both inputs), plus a desired signal at 1 millivolt peak-to-peak (differentially connected to one input). The AM 502 gain is set at 200. The output of the AM 502 shows the desired signal at an amplitude of 0.2 volt (1 millivolt X 200), and the 60 hertz signal is viewed at an amplitude of 0.02 volts. The cmrr in this application is 100,000 to 1. This figure is calculated by multiplying the value of the common-mode signal (10 volt) by the gain of the amplifier (200) for a product of 2000 volts. This product is divided by the observed 60 hertz voltage at the output (0.02 volt) and the result it the cmrr, 100,000 to 1. It would, of course be impossible to view the 1 millivolt signal superimposed on the 60 hertz signal by using single input methods.

    above excerpt from AM 502 DIFFERENTIAL AMPLIFIER Instruction Manual

    1-3366-0494-00Knob, Gray (DC Bal)
    1-4366-1518-00Knob, Gray (Offset-Fine (Inner))
    1-4366-1084-00Knob, Gray (Offset-Fine (Inner))
    1-4366-1023-00Knob, Gray (Offset-Fine (Inner))
    1-5366-1101-00Knob, Gray (Offset-Coarse)
    1-6366-1317-00Knob, Red (Cal)
    1-7366-1001-00Knob, Gray,RF ATTENDB (Gain)
    1-8366-1163-00Knob: Light Gray (HF, LF)
    1-9366-1257-11Push Button:Gray--AC PRE (AC/DC)
    1-10366-1257-12Push Button:Gray--CHG GND (GND)
    1-11366-1489-31Push Button:Gray--DIVIDE 100
    1-67 (S10,20)260-1207-00Switch, Push:2 Module (AC/DC/GND)
    1-69 (S30)260-1209-00Switch, Push:4PDT (divide 100)
    R158311-0467-00Res,Var,Nonwir:100K Ohm,20%,0.50W (CTS 300SF-41334) (DC BAL)
    R178/S178311-1494-00Res,Var,Nonwir:2K Ohm,10%,1W 9 (A-B 11M187) (Cal)
    R468311-0887-00Res,Var,Nonwir:50K Ohm,10%,0.50W (DC Offset-Coarse)
    R467311-1057-00Res,Var,Nonwir:5K Ohm,3%,0.01% (Bourns 3540S-420-502) (DC Offset-Fine)
    U910156-0067-00Microcircuit,LI:Operational Amplifier (Tektronix)
    U990156-0067-00Microcircuit,LI:Operational Amplifier (Tektronix)