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Introduction
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Programmable Gain.
Note The AD1221 and AD1220 do NOT have programmable
gain, the input gain is fixed at 1.
Before the selected input channel is routed to the A/D
converter it passes through the PGA, Programmable Gain
Amplifier. This amplifies the input signal before the conversion
process beings. The programmable gain comes in two varieties:-
one for LOW input signals, eg AD1211L, that has gains of 1,
10, 100 and 1000 and one for HIGH input signals eg AD1211H,
that has gains of 1, 2, 4 and 8.
In this way input voltages from 0 to 10 Volts on one
channel and 0 to 10 milli Volts on another channel can both be
converted to a 12 bit signal resolving across their whole range.
The gain applied to the input is selected, along with the input
channel, by writing to the A/D Gain Channel Register, ADGCR,
register 1.
Programmable gain is a powerful feature enabling the AD1200
series to convert input signals of widely different levels, making
the cards very flexible and expanding the number of possible
applications vastly.
The settling time, to within 0.01%, for the programmable
gain amplifier is 2 microseconds for gains of 1, 2, 4, 8, 10 and
100 and 10 microseconds for a gain of 1000.
Thus the total over all settling, to within 0.01%, time when
both analogue multiplexer and programmable gain amp are
changed is 4 microseconds for gains 1 to 100 and 10
microseconds for a gain of 1000.
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Analogue Input Ranges.
The full scale analogue input ranges are set by jumpers
when the card is installed, unlike the gain applied to the signal
or the input channel routed to the A/D converter which is
software selectable. Please ensure that the GAIN and Analogue
Input Range selected do not result in off scale signals being
applied to the A/D converter. Thus when jumper set for +2.5
Volt inputs, do not use an input voltage of 10 milliVolts with a
Chapter 1
AD1200 Reference
Page 11
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