What is the Gatti Jumper for?

The 7164, 7166, 7167, 7186 and 7187 incorporate a sliding scale method devised by Prof. Emilio Gatti*  for excellent differential linearity characteristics from the ADC circuitry.  Each module has a "Gatti Jumper" option for enabling this circuitry.  This method spans various ADC conversion regions for the same analog input value, thereby reducing the differential non-linearity of the actual ADC.

An output generated by a 6 bit counter is converted to an analog voltage by a DAC.  This analog voltage is subtracted from the incoming data before the ADC conversion.  The analog voltage generated from the DAC is bit for bit equivalent to the digital scale of the ADC.  The value of the counter is then digitally added after the conversion to the ADC output, restoring it to the original input value. This is the value sent to the threshold comparator stage.  The 6 bit counter is incremented each time an ADC cycle occurs.

If the sliding scale Gatti circuit is enabled, it reduces slightly the dynamic range of the ADC.  The 12-bit digital output is valid from channels 64 to 4095.   This happens because below 64 counts, if you subtract an analog voltage that causes the input of the ADC to go below 0 volts, the minimum digital output from the ADC will be 0 volts.  When adding back the digital equivalent of the DAC's analog voltage, it will be greater than the original data.  Therefore any data between 0 and 64 counts should be ignored.  If data is required in this range, the Gatti circuit should be disabled.

The following charts show the "Gatti Jumper" positions for each of the modules:  It should be disabled when  working within the bottom 1.5% of the modules range, or for measurements not creating histograms of data.

7164  ( This module has the option for 6 bits (64 channels) or 7 bits (128 channels) of dithering)
J900
 		J901
 Function
IN
 IN
 Gatti Circuit Disabled
IN
 OUT
 6 Bit Gatti Circuit Enabled  (Lower 64 channels)
OUT
 IN
 For Test Use Only
OUT
 OUT
 7 Bit Gatti Circuit Enabled (Lower 128 channels)

7166, 7167, 7186, 7187
J900
 Function
IN
 Gatti Circuit Disabled
OUT
 6 Bit Gatti Circuit Enabled (Lower 64 channels)

* E. Gatti, V. Svelto, C. Cottini
   "A new method of analog to digital conversion"   Nuclear Instruments and Methods,  Vol. 24,  p.241, 1963.
   "A sliding scale analog to digital converter for pulse height analysis",  Nov. 1963.

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