Generating a ±10.24 V True Bipolar Input for an 18-Bit, 1 Msps SAR ADC
Linear Technology » LTC2338, LTC2338-18, LT1469
Design Note DN522
The LTC2338 is an 18-bit fully differential SAR ADC that is remarkably easy to drive. This 1 Msps ADC operates from a single 5 V supply and achieves ±4 LSB INL maximum with 111 dB THD and 100 dB SNR. Its fully differential ±20.48 V true bipolar input range minimizes the need for range scaling, and its 2k resistive input greatly reduces the charge kickback from the internal sampling capacitor.
ADCs claiming similar performance require scaling to exceed what is typically a 0V to VREF input range, resulting in low impedance inputs or an additional buffer stage requirement. To band limit noise and minimize disturbances reflected into the buffer from sampling transients, other ADCs require filter circuitry composed of expensive film or C0G capacitors at the driver output. In contrast, the simple driver circuit presented here requires only a dual precision op amp and two resistors to drive the LTC2338-18. Layout strategies for this circuit are also shown.
The circuit uses only the LT1469 dual precision op amp and two metal film resistors to form a single-ended to differential driver for the LTC2338-18. This circuit takes a single-ended ±10.24 V input voltage and converts it to the ±20.48 V fully differential signal, which is required for proper operation of the LTC2338-18.
Typical offset for the driver portion of this circuit is less than the equivalent of 1 LSB (156 µV) for the LTC2338-18. Typical AC performance for this circuit includes THD of 110 dB and SNR of 100 dB. The THD and SNR performance are similar to the typical performance numbers found in the LTC2338-18 data sheet - this simple driver produces negligible performance degradation.
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