7.3.2 Reference Buffer Module

On the CONVST rising edge, the device moves from ACQ state to CONV state, and the internal capacitors are switched to the REFBUFOUT pins as per the successive approximation algorithm. Most of the switching charge required during the conversion process is provided by external decoupling capacitor C_REFBUF. If the charge lost from the C_REFBUF is not replenished before the next CONVST rising edge, the voltage on REFBUFOUT pins is less than V_REFBUFOUT. The subsequent conversion occurs with this different reference voltage, and causes a proportional error in the output code. The internal reference buffer of the device maintains the voltage on REFBUFOUT pins within 0.5-LSB of V_REFBUFOUT. All the performance characteristics of the device are specified with the internal reference buffer and specified values of C_REFBUF and R_ESR.

In burst-mode of operation, the device stays in ACQ state for a long duration of time and then performs a burst of conversions. During the acquisition state (ACQ), the sampling capacitor (C_S) is connected to the differential input pins and no charge is drawn from the REFBUFOUT pins. However, during the very first conversion cycle, there is a step change in the current drawn from the REFBUFOUT pins. This sudden change in load triggers a transient settling response in the reference buffer. For a fixed input voltage, any transient settling error at the end of the conversion cycle results in a change in output codes over the subsequent conversions, as shown in Figure 33. The internal reference buffer of the ADS89xxB, when used with the recommended values of C_REFBUF and R_ESR, keeps the transient settling error at the end of each conversion cycle within 0.5-LSB. Therefore, the device supports burst-mode of operation with every conversion result being as per the datasheet specifications.

Figure 33. ADC Output Codes in Burst-Mode Operation With Various ADC Reference Buffers

Figure 34 shows the block diagram of the internal reference buffer.

Figure 34. Internal Reference Buffer Block Diagram

The input range for the device is set by the external voltage applied at the REFIN pin (V_REF). The REFIN pin has electrostatic discharge (ESD) protection diodes to the RVDD and GND pins. For minimum input offset error (see E_(IO) specified in the Electrical Characteristics), set the REF_SEL[2:0] bits to the value closest to V_REF (see the OFST_CAL register).

The internal reference buffer has a typical gain of 1 V/V with minimal offset error (see V_(RO) specified in the Electrical Characteristics), and the output of the buffer is available between the REFBUFOUT pins and the REFM pins. Set the REF_OFST[4:0] bits to add or subtract an intentional offset voltage (see the REF_MRG register).

Figure 35 shows the external connections required for the internal reference buffer.

Figure 35. External Connections for the Internal Reference Buffer

Select R_{REF_FLT} and C_{REF_FLT} to limit the broadband noise contribution from the external reference source. The device takes very little current, I_REF, from the REFIN pin (typically, 0.1 µA). However, this current flows through R_{REF_FLT} and may result in additional gain error.

Short the two REFBUFOUT pins externally. Short the two REFM pins to GND externally. As shown in Figure 35, place a combination of R_ESR and C_REFBUF (see the Electrical Characteristics) between the REFBUFOUT pins and the REFM pins as close to the device as possible. See the Layout section for layout recommendations.