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a101 photo - 10k

Designed for direct applications in the field of aerospace instrumentation, mass spectrometers, laboratory and research experiments, medical electronics, and electro-optical systems.

Model A101 is a hybrid charge sensitive preamplifier, discriminator, and pulse shaper developed especially for instrumentation employing photomultiplier tubes, channel electron multipliers and other low capacitance charge producing detectors in the pulse counting mode. While this unit was specifically designed for satellite instrumentation, the following unique characteristics make it equally useful for space, laboratory and commercial applications:




Vs = +5 V, T = +25 °C
ThresholdModel A101 has a nominal threshold referred to the input of 1.6 x 10-13 coulomb. This is equivalent to 106 electrons. The threshold can be increased by the addition of a resistor between Pin 9 and 12. See Figure 1.
Stability< 1.5% of threshold, 0 to +50 ºC. See Figure 3.
NoiseRMS noise level < 0.4% of threshold.
ProtectionBack-to-back diodes to ground.
Detector Capacitance0 to 60 pF

Output Characteristics

1) Pin 5 provides a positive 5 volts output pulse capable of interfacing directly with CMOS.

Pulse characteristics are:

Risetime6 ns
Falltime20 ns
Width220 ns nominal. May be increased to greater than 1 µs by the addition of a capacitor between Pin 3 and 4. See Fig. 2.
Amplitude5 volts

2) Pin 6 is an open collector output and with an external pullup resistor provides a negative going pulse (the complement of Pin 5). This output can drive TTL and can be wireor'ed with other units. Pullup resistor may be connected to Vs or to other positive supply up to +15 V.


Count Rate4 x 106 CPS, Periodic
Pulse Pair Resolution250 ns
Operating Voltage+4 to +10 VDC
Operating Current3 mA quiescent, 4 mA @ 105 CPS
Temperature-55 to +70 ºC operational
WarrantyOne year
Package12-Pin, TO-8 case.
ScreeningAmptek High Reliability
Test BoardPC-11

Operating Notes

Power Requirements

All device specifications apply to operations at +5 V. The unit can be operated up to +10 V with increased power dissipation and threshold. Supply is internally bypassed. Care should be taken in the circuit layout and in some applications power supply decoupling may be helpful.

The case is internally connected to Pin 1, ground.


The detector is normally capacitively coupled to Pin 12 with a capacitor of adequate voltage rating. Alternately, the detector anode can be direct coupled to the input if the cathode is at negative potential. The A101 is sensitive to a negative charge pulse of at least 0.16 picocoulomb. This threshold may be increased by the connection of a resistor between Pins 9 and 12. Approximate values are given in Figure 1.

Figure 1.

The A101 can be tested with a pulser by using a small 2pF test capacitor to inject a test charge into the input. The unit will trigger on the negative-going edge of the pulse which should have a much longer fall time (> 1 µs) or a square wave may be used. (If a square wave is used, triggering on the positive-going edge will occur for large pulses.)

Charge transfer in the test circuit is according to Q = C·V where Q = total amount charge, C = capacitance, and V = voltage. Use only a small capacitor in this circuit (1 to 10 pF). DO NOT connect a low impedance pulser through 500 pF when testing as this will produce a large pulse through the input transistor and may cause irreversible damage.

Tr < 20 ns, Tf > 1 µs
Negative going
Amplitude: 0.25 V = 0.5 picocoulomb


The output at Pin 5 is a positive, 220 ns wide pulse.

Pin 6 is an open collector output and should be left unconnected if not used. A negative going output can be obtained by connecting a pullup resistor (typically 1 kohm) between Vs and Pin 6. The pullup resistor may be tied to any positive voltage up to 15 V (as required for example, in interfacing with particular logic families.) In multi-detector systems, where more than one preamp drives the same counter, the open collector outputs of several A101s can be connected together to the counter input with a single pullup resistor. DO NOT CONNECT THIS PIN DIRECTLY TO Vs (PIN 2) AS THIS MAY DAMAGE OUTPUT STAGE AND VOID WARRANTY.

The output pulse with can be increased by the connection of a capacitor between Pin 3 and 4. See Figure 2 for typical values.

Figure 2.

If the standard pulse width is adequate Pin 3 and 4 must be left unconnected.

Figure 3.

Line Driver for A101


Due to its hermetic seal and small size, the A101 is well suited for use within a vacuum chamber. In such applications the mounting should provide adequate heat dissipation and care should be taken to avoid electrical discharge near the input, which can damage the unit and void the warranty.

Use care in soldering leads - avoid overheating.

Pin Assignments

Pin 1 Ground
Pin 2 Vs (+4 to +10 VDC)
Pin 3 Pulse Width Adjust
Pin 4 Pulse Width Adjust
Pin 5 Positive Output
Pin 6 Open Collector Output
Pin 8 No Connection
Pin 8 No Connection
Pin 9 Threshold Adjust
Pin 10 No Connection
Pin 11 No Connection
Pin 12 Input

PC-11 Test Board for the A101

The PC-11 is a printed circuit board designed to facilitate testing of the A-101. In addition to testing circuitry, it provides component locations for use with detectors. Ground plane construction minimizes external pick-up.

[pc-11 layout]
Dimensions: 1.75 in X 1.75 in (4.45cm X 4.45cm)

INDetector Input; Pin 12; should be AC coupled with a high voltage capacitor (500-1000pF)
DETProvides post to connect the detector and input capacitor
TEST INInput to test circuit as described in specifications
VsPin 2; supply voltage (+4 to +10 VDC)
H.V.Provides post to connect the detector to the high voltage supply through a resistor
+OUTPositive, TTL type output from Pin 5
O.C. OUTNegative; open collector output from Pin 6 (must be connected through 1 kohm to Vs)
BUF OUTPositive output through a Buffer/Line Driver IC from Pin 5
CvFilter capacitor (1uF)
RpPullup resistor (1 kohm)
CTest capacitor (2 pF)
RTest pulse termination resistor (50 ohm)
RtThreshold adjust resistor
CwPulse width adjustment capacitor
CdHigh voltage detector coupling capacitor (user supplied)
RbDetector Bias resistor (user supplied)
U2Line driver TPS2829

[pc-11 schematic]

PC-11 in PDF format (426 k)

Connection of a photomultiplier tube to the A-101 Using + High Voltage

Connection of a Channel Electron Multiplier to the A101

Connection of a photomultiplier tube to the A-101 Using - High Voltage

A-101 Specifications in PDF format (1.4 Meg)

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Revised September 8, 2003