RAMP

TEK

0.145

Amptek’s BLR1 restores thebaseline in high counting rateapplications. The BLR1 istypically used with the A275,
A203, and A225. In order tosimplify testing and prototype
work, the PC-275 test board isalso available.

0.075
0.150

0.400
10.017(inches)

SIDE VIEW

81-6 V

4120.500

10 k

CTOP VIEW

EXTERNAL

10 k

OUTPUT
12668 nF
CASEandGROUND+6 V
97
BLR1 PIN CONFIGURATIONPIN

FUNCTION

1External capacitor to ground (as necessary to achieve loop stability ~10 μF)

4+ Input6- Input

7+6 V (At 2.5 ma)

8-6 V (At 2.5 ma)

9Ground & case

12Output (If the unit oscillates, use Pin 1 for Output with an external 10 K, replacing
the internal 10 K.)

2, 3, 5, 10, 11

No connection

THEORY OF OPERATIONThe BLR1 is a transconductance amplifier with transconductance of approximately 17ma/Volt andvery wide bandwidth. In operation, it amplifies both the DC baseline component and the pulse signal
component by a large factor. Typically, with the PIN 12 output connected to a low impedance, theamplifier load is the internal 20kΩ resistor. The low frequency voltage gain at the amplifier output is17ma/Volt x 20kΩ = 340. The amplified pulses are clipped by the pair of Schottky diodes to an
amplitude of ±0.5V and thus have little influence on the average output of the amplifier, which isdominated by the amplified baseline. This output is then passed through a low-pass RC filter formedby the first 10kΩ and the total capacitance (internal and external) from PIN 1 to ground. The filteredoutput is fed back to the input of the shaping amplifier chain to zero the baseline at the output of theamplifier chain.
EXAMPLE
Assume both BLR1 and the amplifier chain have zero offset voltage, so that in the absence of pulses theBLR1 output is zero, since no correction is required. Now assume 1μs pulses are present at the output of
the amplifier chain at a rate of 105/s. At the output of the BLR1 amplifier, this will produce a 10% duty cycle
waveform with peak amplitude of 0.5V. In order that the average of this signal be near zero volts, thebaseline at this point will have to shift downward by (0.5V ÷ 9) = 56mV. Dividing by the amplifier gain of340, we get a baseline shift at the input of the BLR1 of just minus 160μV. To generalize:
0.5V (twn)Baseline Shift = ——————————
AV (1 - twn)
where:

tw = pulse width (near baseline)· n = pulse rate (c.p.s.)
AV = BLR1 amplifier voltage gain (normally about 340)

Note that when the duty cycle reaches 50%, the baseline shift at the output of the BLR1 amplifier
will reach -0.5V, at which point negative clipping will occur and the baseline can no longer bemaintained. This can be considered to be BLR “saturation.”

The above example is an ideal case. In a real case, there are several complicating factors which must
be considered:
1) There is not a single pulse width, since the effective width is dependent on amplitude,with a larger pulse appearing to be wider. Thus, the pulse height spectrum influences
the saturation count rate.
2)
The statistical variation in counting rate will affect the baseline shift.
3)
The noise is also amplified by the BLR1 amplifier and appears at the output. Normally,
this noise should be well below the clipping voltage. However, at high count rates, as thebaseline approaches the negative clipping level, negative noise peaks will be clipped,
which will cause the baseline to begin to shift sooner than in the ideal case. Using a long
low-pass time constant minimizes these effects, at the expense of slower recovery should
saturation occur.
Since saturation is dependent on duty cycle, using a shorter peaking time, or pulse width, in theshaping amplifier will proportionally increase the saturation count rate. Too short of a peaking time
however, will increase the overall noise of the system.
RAMP

TEK

AMPTEK, INC.

14 DeAngelo Drive, Bedford, MA 01730-2204 USA
TEL: +1 (781) 275-2242 FAX: +1 (781) 275-3470
email: sales@amptek.com www.amptek.com