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A low noise, large dynamic range pulse amplifier
Nuclear Instruments and Methods 176 (1980) 283-286 © North-Holland Publishing Company
A LOW NOISE, LARGE DYNAMIC R A N G E PULSE AMPLIFIER
J. COLAS and J.C. LACOTTE Laboratoire de Physique des Particules, B.P. 909, Annecy le Vieux, France
We have developped a low noise, high dynamic range low cost amplifier. This amplifier will equip the shower position detector (current division readout) and the photomultipliers of the forward electromagnetic calorimeter in the UA1 experiment (CERN ffp collider).
1. Introduction
2. The amplifier
We are building lead-scintillator calorimeters to measure the total electromagnetic energy of photons and electrons to be observed at the PP collider in the UA1 experiment [1]. A position detector built with proportional tubes read out by current division, will be placed inside the calorimeters at a depth of 11 radiation lengths [2]. The dynamic range to be covered by the electronics of such a detector is quite large from a single penetrating particle to a 200 GeV electron which showers in a thousand of low energy electrons. The lowest signal to be processed by the electronics is quite small, as the gain of the detector itself is low to not saturate the detector for a high energy shower (for instance gas amplification in the proportional tubes around 103). We thus end up with the following requirements for the preamplifier: - l a r g e dynamic range (104) with two overlapping outputs to fed two different ADCs, low gain: G ~ 25, high gain: G --~ 750, linear output signal up to at least 2500 pC; - low equivalent input noise (<104 electron charges); - g o o d linearity over the whole range (AG/G ~ 1%) and good gain stability; - the output signal should be fully inside a 250 ns ADC gate; the power dissipation should be low and the cost minimal as we need to equip 2000 channels. After several tries, we end up with the amplifier described in this article. Although simple, this amplifier exhibits good performance. More than hundred units have been built and all work satisfactorily.
Most of the low noise current amplifiers follow the same lines. The input current is converted to a voltage via a gain resistor. The output is driven to that voltage by a low impedance source. The current gain is then the ratio of the gain resistor to the load impedance. Fig. 1 sketches the circuitry we use. A common base transsistor transfers the input current through the gain resistor in the collector (fig. 1, part A). The resulting voltage is fed to the twisted pair driver by a high input impedance and highly linear voltage follower (fig. 1, part B). This stage provides a natural split between the low gain and high gain channels. This voltage follower exhibits good linearity. Indeed the first transistor of this stage acts only as a voltage comparator and does not have to provide the output current. Thus the non-linearity of the base emitter voltage with the emitter current does not affect the linearity performance o f the voltage follower.
2. Performance 2.1. Pulse shapes
Fig. 2b (c) shows the negative and positive output pulses of the low gain (high gain) channel when the amplifier is fed by the test input signal shown in fig. 2a (attenuated by 22.7). The positive and negative pulses are quite symmetric. They follow the input signal with a slightly degraded rise and fall time. Good linearity performance is obtained up to output pulses of nearly 2 V. 283
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285
Fig. 3. Low gain and high gain outputs corresponding to the test input signal and showing the high gain channel saturation.
2.2. L o w gain/high gain splitting Low gain and high gain channels are well decoupied (fig. 3). The high gain channel may saturate without affecting the response of the low gain one.
2.3. Noise
(b)
As already stressed before, the noise of the amplier is an important parameter. When the high gain output is plugged into an ADC, the amplifier noise gives a width to the pedestal. Fig. 4 shows the pedestal of
• .......... •
. ~
(c) Fig. 2. (a) Test input signal. (b) Low gain outputs. (c) High gain outputs (input attenuated by 22.7).
Fig. 4. Pulse height spectrum corresponding to a zero charge signal (pedestal) and to a 50 000 electron signal input to the high gain amplifier (ADC gate length 250 ns). VI. ELECTRONICS
286
J. Colas, J.C Lacotte / Pulse amplifier Pcdc~;t:ll t.:idLI ADC ~ ! l , l n l / ~ , l
(:)
4 4
l\i)(~
gate
len,~th
(as)
Fig. 5. Pedestal r.m.s, vs ADC gate length. The curve is the computed contribution due to the theoretical noise of the first stage.
the ADC and the response of the electronic chain to an input signal of 50 000 electrons. The two peaks are well separated. Fig. 5 shows the variation o f the noise with the ADC gate length. These results are quite close to what one can compute from the shot noise o f the input transistor and from the thermal noise of the resistors which gives the dominant contribution. For a 250 ns gate, we have an equivalent input noise of ~ 7 5 0 0 electron charges. 2.4. L inearity The gains o f the two stages o f the amplifier were measured using input signals scaled down by precision attenuators and covering the whole dynamic range (from 0.01 to 100 pC). No significant gain variation was observed (AG/G ~ 2%). The measurement errors are dominated by the precision on the attenuators. A more refined measurement has been made by Strauss [3] using a precise pulse generator. He quotes a differential linearity better than 1% over the whole range.
3. Conclusion The amplifier described here fulfills the requirements we had. It is cheaper than what we have found commercially available. Thirty-two o f these amplifiers have been used successfully in a test experiment [2]. For the CERN UA1 p r o t o n - a n t i p r o t o n experiment, sixteen such amplifiers will be implemented on a single Camac board. This card will include also a precision pulser used to calibrate accurately each channel.
References [1] Aachen-Annecy-Birmingham-CERN-Collt~ge de FranceQueen Mary College-Riverside-Rutherford-Saclay Collaboration, CERN SPSC/78-06. [2] B. Aubert, P. Catz, J. Colas, M. Della-Negra, A. Gonidec, J.P. Lees, D. Linglin, M.N. Minard and M. Yvert, these proceedings, p. 195. [3] J. Strauss, private communication.
A LOW NOISE, LARGE DYNAMIC R A N G E PULSE AMPLIFIER
J. COLAS and J.C. LACOTTE Laboratoire de Physique des Particules, B.P. 909, Annecy le Vieux, France
We have developped a low noise, high dynamic range low cost amplifier. This amplifier will equip the shower position detector (current division readout) and the photomultipliers of the forward electromagnetic calorimeter in the UA1 experiment (CERN ffp collider).
1. Introduction
2. The amplifier
We are building lead-scintillator calorimeters to measure the total electromagnetic energy of photons and electrons to be observed at the PP collider in the UA1 experiment [1]. A position detector built with proportional tubes read out by current division, will be placed inside the calorimeters at a depth of 11 radiation lengths [2]. The dynamic range to be covered by the electronics of such a detector is quite large from a single penetrating particle to a 200 GeV electron which showers in a thousand of low energy electrons. The lowest signal to be processed by the electronics is quite small, as the gain of the detector itself is low to not saturate the detector for a high energy shower (for instance gas amplification in the proportional tubes around 103). We thus end up with the following requirements for the preamplifier: - l a r g e dynamic range (104) with two overlapping outputs to fed two different ADCs, low gain: G ~ 25, high gain: G --~ 750, linear output signal up to at least 2500 pC; - low equivalent input noise (<104 electron charges); - g o o d linearity over the whole range (AG/G ~ 1%) and good gain stability; - the output signal should be fully inside a 250 ns ADC gate; the power dissipation should be low and the cost minimal as we need to equip 2000 channels. After several tries, we end up with the amplifier described in this article. Although simple, this amplifier exhibits good performance. More than hundred units have been built and all work satisfactorily.
Most of the low noise current amplifiers follow the same lines. The input current is converted to a voltage via a gain resistor. The output is driven to that voltage by a low impedance source. The current gain is then the ratio of the gain resistor to the load impedance. Fig. 1 sketches the circuitry we use. A common base transsistor transfers the input current through the gain resistor in the collector (fig. 1, part A). The resulting voltage is fed to the twisted pair driver by a high input impedance and highly linear voltage follower (fig. 1, part B). This stage provides a natural split between the low gain and high gain channels. This voltage follower exhibits good linearity. Indeed the first transistor of this stage acts only as a voltage comparator and does not have to provide the output current. Thus the non-linearity of the base emitter voltage with the emitter current does not affect the linearity performance o f the voltage follower.
2. Performance 2.1. Pulse shapes
Fig. 2b (c) shows the negative and positive output pulses of the low gain (high gain) channel when the amplifier is fed by the test input signal shown in fig. 2a (attenuated by 22.7). The positive and negative pulses are quite symmetric. They follow the input signal with a slightly degraded rise and fall time. Good linearity performance is obtained up to output pulses of nearly 2 V. 283
VI. ELECTRONICS
~> c~ " ~
=g ~.~ +
8 g,
g -
1
1 121~
H~" b~
~
Zo 3.3K
3.OK
< 2~7 K r-~ C~
•
.~
~
C~
i
.o~- ~ il ~
56
--4
56
56
56
c
0
aa!f!ld w v aSlnd / alloov7 "DY 'SVlOD f
t78~
J. Colas, J.C. Lacotte /Pulse amplifier
(a)
285
Fig. 3. Low gain and high gain outputs corresponding to the test input signal and showing the high gain channel saturation.
2.2. L o w gain/high gain splitting Low gain and high gain channels are well decoupied (fig. 3). The high gain channel may saturate without affecting the response of the low gain one.
2.3. Noise
(b)
As already stressed before, the noise of the amplier is an important parameter. When the high gain output is plugged into an ADC, the amplifier noise gives a width to the pedestal. Fig. 4 shows the pedestal of
• .......... •
. ~
(c) Fig. 2. (a) Test input signal. (b) Low gain outputs. (c) High gain outputs (input attenuated by 22.7).
Fig. 4. Pulse height spectrum corresponding to a zero charge signal (pedestal) and to a 50 000 electron signal input to the high gain amplifier (ADC gate length 250 ns). VI. ELECTRONICS
286
J. Colas, J.C Lacotte / Pulse amplifier Pcdc~;t:ll t.:idLI ADC ~ ! l , l n l / ~ , l
(:)
4 4
l\i)(~
gate
len,~th
(as)
Fig. 5. Pedestal r.m.s, vs ADC gate length. The curve is the computed contribution due to the theoretical noise of the first stage.
the ADC and the response of the electronic chain to an input signal of 50 000 electrons. The two peaks are well separated. Fig. 5 shows the variation o f the noise with the ADC gate length. These results are quite close to what one can compute from the shot noise o f the input transistor and from the thermal noise of the resistors which gives the dominant contribution. For a 250 ns gate, we have an equivalent input noise of ~ 7 5 0 0 electron charges. 2.4. L inearity The gains o f the two stages o f the amplifier were measured using input signals scaled down by precision attenuators and covering the whole dynamic range (from 0.01 to 100 pC). No significant gain variation was observed (AG/G ~ 2%). The measurement errors are dominated by the precision on the attenuators. A more refined measurement has been made by Strauss [3] using a precise pulse generator. He quotes a differential linearity better than 1% over the whole range.
3. Conclusion The amplifier described here fulfills the requirements we had. It is cheaper than what we have found commercially available. Thirty-two o f these amplifiers have been used successfully in a test experiment [2]. For the CERN UA1 p r o t o n - a n t i p r o t o n experiment, sixteen such amplifiers will be implemented on a single Camac board. This card will include also a precision pulser used to calibrate accurately each channel.
References [1] Aachen-Annecy-Birmingham-CERN-Collt~ge de FranceQueen Mary College-Riverside-Rutherford-Saclay Collaboration, CERN SPSC/78-06. [2] B. Aubert, P. Catz, J. Colas, M. Della-Negra, A. Gonidec, J.P. Lees, D. Linglin, M.N. Minard and M. Yvert, these proceedings, p. 195. [3] J. Strauss, private communication.