Fixed Threshold

The thresholds of the AD1 and AD6 are factory set to 1 mV and 2 mV, respectively.

These are appropriate thresholds for optimal photomultiplier performance, requiring a gain of only 106.

Variable Threshold

The AD2, AD3, and AD7 have adjustable thresholds. This gives an advantage where setting a higher threshold to eliminate pickup from an electrically noisy environment.

The input to the boxed amplifier-discriminators is made via a BNC coaxial connector with an input impedance of 50 Ω.

Connections should be made via 50 Ω matched cable to ensure maximum count rate capability and minimum reflections.

The cable length should be kept to a minimum.

TTL Output

TTL signals are the most common mode adopted for electronic modules.

These amplifier-discriminators have a 50 Ω output impedance delivering +5 V un-terminated, or 2.5 V into a 50 Ω load.

The AD6 and AD7 have TTL outputs compatible with the requirements of the CT2 counter-timer. A complete PC, or laptop, controlled photon counting system can be readily configured in this way. Please refer to our page on system architecture.

TTL Output

ECL Output

The ECL output driver provides complementary outputs via a twisted pair of wires usually into a differential receiver. The high state is nominally 1 V, and the low state 2 V, and vice versa for the complementary output.

The advantages of this logic mode are immunity from ground loops and noise interference. The AD1, AD2, and AD3 have this type of output.

Pulse Pair Resolution

Every discriminator circuit has an intrinsic dead time. The pulse pair resolution is the minimum time before a second pulse will be registered. Please refer to our brochure Understanding Photomultipliers for further information.

Count Rate Correction

Maximum dynamic range can be realised by applying a count range correction to compensate for departure from linearity at high count rates due to pulse pile up.

True count rate N = n/(1-nT)

Where n is the measured count rate and T, the correction factor, is equal to the pulse pair resolution given in the amplifier discriminator selection guide here.

Count Rate Correction graph

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