BLR1: Baseline restorer. Company advantage inventory.

theory of operation

BLR1 is a transconductance amplifier, its transconductance is about 17 ma/Volt,

very wide bandwidth. In operation, it amplifies the DC baseline component and the pulsed signal

a large part of the factor. Typically, after connecting the PIN 12 output to a low impedance,

The amplifier load is an internal 20kΩ resistor. Low Frequency Voltage Gain at Amplifier Output

is 17 ma / Volt x 20kΩ = 340. The amplified pulse is clipped by a pair of Schottky diodes to a

The amplitude is ±0.5 V, so it has little effect on the average output of the amplifier

Based on the augmented baseline. This output is then passed through a low-pass RC filter that forms

Add the first 10kΩ and the total capacitance (internal and external) from PIN 1 to ground. after filtering

The output is fed back to the input of the shaping amplifier chain so that the baseline of the output is zero.

Amplifier chain.

example

Assuming both the BLR1 and the amplifier chain have zero offset voltage, so in the absence of pulses,

The BLR1 output is zero because no correction is required. Now suppose a 1 μs pulse appears at the output

Zoom in at a rate of 105

/s. At the output of the BLR1 amplifier it will produce a 10% duty cycle

Periodic waveform with a peak amplitude of 0.5V. In order to bring the average value of this signal close to zero volts,

At this point the baseline must move down (0.5 V ÷ 9) = 56 mV. divide by the amplifier

With a gain of 340, the baseline shift we get at the input of BLR1 is only negative 160 μV. Summarized as follows:

0.5V (twn)

Baseline Offset = ———————————

?AV (1-twn)

where: tw = pulse width (near baseline)

n = pulse rate (cps)

?AV = BLR1 amplifier voltage gain (typically around 340)

Note that when the duty cycle reaches 50%, the baseline shift of the BLR1 amplifier output

will reach -0.5 V, at which point there will be negative clipping and the baseline will no longer

Keep. Think of it as BLR "saturated".

The above example is an ideal situation. In practice, there are several complicating factors

Must consider:

?1) There is no single pulse width, since the effective width depends on the amplitude,

Larger pulses appear to be wider. Therefore, the pulse height spectrum affects

Saturation count rate.

2) Statistical changes in count rate will affect the baseline shift.

3) Noise is also amplified by the BLR1 amplifier and appears at the output. generally,

This noise should be well below the clipping voltage. However, at high count rates,

The baseline is close to the negative clipping level, the negative noise peaks will be clipped,

This will cause the baseline to start shifting sooner than ideal. use one

A long low-pass time constant minimizes these effects, but at the expense of slower recovery

Saturation should occur.

Since saturation depends on duty cycle, use shorter peak times or pulse widths in shaping

The amplifier will proportionally increase the saturation count rate. However, the peak hours are too short,

Will increase the overall noise of the system.