Generating a ±10.24V True Bipolar Input for an 18-Bit, 1Msps SAR ADC

Introduction

The LTC2338 is an 18-bit fully differential SAR ADC that is remarkably easy to drive. This 1Msps ADC operates from a single 5V supply and achieves ±4LSB INL maximum with –111dB THD and 100dB SNR. Its fully differential ±20.48V true bipolar input range minimizes the need for range scaling, and its 2kΩ 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 V_REF 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.

Simple Driver Circuit

The circuit of Figure 1 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 ±10.24V input voltage and converts it to the ±20.48V 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 1LSB (156μV) for the LTC2338-18. Typical AC performance for this circuit includes THD of –110dB and SNR of 100dB. This performance can be seen in the FFT of Figure 2. 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.

 

 

 

 

Layout Is Important

PC board layout can have a significant effect on the performance of a high speed 18-bit ADC. When considering layout, keep the following in mind:

• A ground plane should always be used—a solid ground plane just below the component layer is recommended.
• Keep traces as short as possible.
• Keep bypass capacitors as close to the supply pins as possible, and each bypass capacitor should have its own low impedance return to ground
• The analog input traces should be screened by ground.
  The layout involving the ADC analog inputs should be as symmetrical as possible, so that parasitic elements cancel each other out.
• The reference bypass capacitors should be as close to the REFBUF and REFIN pins as possible.

Figure 3 shows a close up of the layout connecting the LT1469 and the LTC2338-18 on a demonstration board. Device, pin and component numbers shown in the photograph of Figure 3 correspond to the numbers shown in the schematic of Figure 1.

Conclusion

The LTC2338-18, with its large true bipolar input voltage range and resistive input, greatly simplifies the task of driving an 18-bit fully differential SAR ADC. Using the simple driver circuit presented here, consisting of only a dual precision op amp and two resistors, it is possible to maintain the good AC and DC specifications of this ADC. PCB layout is an important consideration in achieving this level of performance. Proper use of a ground plane, keeping bypass capacitors near pins being bypassed and symmetrical layout around the analog inputs help ensure a high level of performance.

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