Question
Can you tell me more about the Adaptive Level Shift (ALS) Technology?
Answer
ALS is an ADI patent-pending technique that we believe will be useful in many
designs, especially for instrumentation amplifiers, but possibly for precision
op-amps and in other spaces as well. Most rail-to-rail-input amplifiers rely on
two pairs of input transistors to reach either rail. There is typically a
crossover distortion effect, where the offset modulates when one of the input
pairs turns off and the other pair turns on. There is only a single PMOS input
differential pair for each of the AD8237’s gm trans conductance amplifiers.
Therefore, the AD8237 relies on the ALS to achieve rail-to-rail input and there
are no undesirable input crossover distortion effects.
Here is the basic process for how ALS works: First, it senses the common-mode
level at the inputs, VIN¬_CM. Then it derives a level-shift voltage, VSHIFT =
VIN_CM – VOPTIMAL, and charges two capacitors to this voltage. These capacitors
are then switched in series with the inputs, so VSHIFT is subtracted from the
voltage at both inputs. This shifted voltage is buffered and made available at
the output of the ALS block. The common-mode voltage at the output of the ALS
block is at the optimal level for the PMOS differential pair. Then the process
is repeated.
After the ALS block the common-mode voltage is equal for the input side and the
feedback side. The cross-coupled connection (shown in the AD8237 data sheet
figure 65) between the ALS output and the gm amplifiers is conceptually a
differential common-mode conversion. With zero common-mode difference
between the two sides, this conversion resolves most of the issues that the ICF
architecture has with differential voltages, which were discussed above.