The WEBENCH ® Series Reference Selector tool assists designers to identify the correct components for the reference circuitry to an analog-to-digital converter (ADC). Initially, the user selects their Texas Instruments ADC from a list of 450+ Precision ADCs (Figure 3). After selecting the converter, this tool provides a list of possible voltage reference devices along with a reference specification table, an Error Summary and the system's power consumption. The user can select their reference device or modify the design input conditions.
In your design, you may be exclusively concerned about the analog input and the digital output of our analog-to-digital converter (ADC). But look beyond these two ports and include the ADC's voltage reference inputs in your list of concerns. Figure 1 shows a simple circuit diagram that you can use to drive an ADC's reference pin.
Figure 1. General voltage reference to SAR circuit diagram.
The circuit in Figure 1 may not be appropriate for all ADCs, but the basic functionality is important to understand. In Figure 1, the Voltage Reference block provides an output voltage value per the ADC's reference specifications. Typically, voltage reference chips will not be able to drive the ADC's reference pin directly because of the dynamic current spikes in and/or out of the ADC's reference pin. Following the Voltage Reference block, there is a low pass filter (RREF_FLT and CREF_FLT). You would design this filter to reduce the higher frequency output noise of the Reference Voltage chip. The output of RREF_FLT drives directly into a buffer configured operational amplifier. The use of this amplifier is to 'isolate' the Voltage Reference chip from the converter. The output of the amplifier connects to the reference input of the ADC and RBUF_FLT in series with CBUF_FLT to ground. RBUF_FLT is very low and usually the ESR resistance of the capacitor. CBUF_FLT provides ample charge to the input of the ADC's voltage reference pin. This configuration stabilizes the reference voltage to the ADC allowing a more stable conversion to occur. After you select your ADC, an appropriate schematic for your ADC will appear on the WEBENCH Series Reference Selector page (Figure 5).
The Series Voltage Reference selection and analysis goes through a two-step approach (Figure 2):
Figure 2. Two-step process for selecting an appropriate reference for a selected ADC.
Step 1: Select ADC
In the Select ADC view (Figure 3), there are four ways to sort through the ADC
- Filter By ADC (TI part number)
- Filter By Resolution (8 to 31 bits)
- Filter By Automotive Grade
- Filter By AVDD (1 to 11V)
- Forward or reverse alphabetize the ADC list (click in "ADC" in table column)
Figure 3. In the "Select ADC" view, you can sort and finally select the ADC that you plan to use in your design.
- If you already know which ADC that you would like to use, you can enter the ADC part number in the “Filter By ADC” box. The table below will reflect your choice. If your device does not appear in this table, this tool does not support your chosen device, generally because the ADC does not use an external reference.
- You can sort the list of ADCs by using the "Filter By Resolution" pull down menu. Devices that have your selected resolution will appear in the table below.
- If your design requires automotive-grade devices, you can check the box in “Filter By Automotive Grade” section. The table will show you the list of only Automotive –grade supported ADCs.
- If you are interested in sorting the ADC in terms of their power supply voltage, use the "Filter By AVDD" slider to center in on your power supply of choice. Devices that meet your selected power supply voltage will appear in the table below.
- You can sort the ADC in the table alphabetically in forward or reverse order.
Once you have found the ADC that you are interested in, click on the "Select" button (Figure 4). Note: If you click a specific part within the ADC column, you will be redirected to the part's product folder.
Figure 4. Highlight the ADC you are interested in and Click on the "Select" button
to proceed to the WEBENCH Series Reference Selector page.
After clicking "Select", the Voltage Reference selection window will appear.
Step 2: Series Reference Selector
Figure 5. WEBENCH Series Voltage Reference Selector View.
There are eight parts in the Voltage Reference selection window (Figure 5):
- Display of the highlighted ADC (Figure 5, area 1)
This area displays the properties of ADC that is selected in the previous step. If you click on the ADC you have selected it is hyperlinked to the product data sheet. The shopping cart symbol will help you to get a sample of the part.
- Change ADC Selection (Figure 5, area 2)
If you want to change the ADC, click on the “Choose Another ADC” button, which will lead you back to the ”Select ADC” view, allowing you to select another ADC.
- System parameters (Figure 5, area 3)
You can view automotive-grade references by checking the box for Automotive References. (This box is checked by default when you select an Automotive-grade ADC.) The system operating temperature range is set at a default range of 20 to 70C. This range has been chosen so that all feasible Voltage Reference devices will be displayed irrespective of their temperature range. Overwrite these temperature values with the ones applicable to your design. Choose carefully, as the Voltage Reference’s accuracy grades are sensitive to the temp range.
The Alternate Reference value is chosen according to the most appropriate voltage reference for your selected converter. You can choose lower value references, as required by your application, with the pull-down menu.
- Series Reference Table of Specifications (Figure 5, area 4)
The ADC Recommended Series Voltage Reference chips are displayed in the table on the right (Figure 5, area 2), based on the ADC’s technical specifications. If you choose, you can select a lower value voltage reference by using the Alternate References pull down menu. If you select from the “Alternate References” pull down menu, another Voltage Reference table will be populated according to your selection.
This table shows a list of feasible Voltage Reference devices, which is based on the selection criteria above (Voltage Reference voltage and temperature range). In this list, all of the feasible Voltage References with their corresponding parameters are displayed. On the left column in this table, there is a selection box. By choosing up to five Voltage Reference devices and clicking on the “Compare Up to 5 Parts” below the table, a comparison report will be generated. This report will compare the reference’s accuracy level and power consumption in convenient bar charts (Figure 8).
- Optimizer knob (Figure 5, area 5)
This knob sorts the list of feasible Voltage References according to three criteria:
Changing the knob only changes the ranking of the respective devices in the Voltage Reference parts list; it does not exclude any parts from further evaluation by the user.
- Initial accuracy
- Voltage Noise
- Temperature Drift
The graph below (Figure 6) shows the weighting function for each of the three objectives: Accuracy, Voltage Noise and Temperature Drift. The horizontal axis indicates the five positions of the dial knob. The vertical axis shows the relative weight of each objective.
Figure 6. Weighted algorithms for the Optimizer Knob.
- Display of the highlighted Voltage Reference (Figure 5, area 6)
This area displays the highlighted Voltage Reference in the voltage reference table. You can click on the part name to reach the product folder or click on the shopping cart to get a sample.
- ADC – Voltage Reference architecture (Figure 5, area 7)
This area displays the corresponding schematic for the ADC chosen. In most cases it uses the SAR architecture (see Figure 1). However there are some ADCs which use different arrangements (Figure 7), e.g. ADS1258.
Figure 7. Voltage Reference to delta-sigma ADC example circuit diagram.
- Error summary (Figure 5, area 8)
This area displays the accuracy of the Voltage Reference highlighted in the voltage reference table.
The “LSB size” of the ADC with respect to the applied Voltage Reference is the first specification listed. The “Reference Error” summarizes this error in milli-volts, the ADC LSB, and the ADC’s % of Full-Scale Range. The “Reference Error After Calibration” summarizes reference error minus the room temperature error.
Error Analysis Window
The view in Figure 8 compares the various types of voltage reference errors in terms milli-volts (mV), the ADC’s LSB (LSB), and the ADC’s percentage of full-scale range (% of FS). Your selection between these units is reflected in the table of specifications and graphically below.
- mV ➝ milli-volt, these are absolute values
- LSB: least significant bit size of the ADC, as determined by Voltage Reference and ADC resolution
- FS: ADC Full-Scale Range
Figure 8. Error report view
The WEBENCH ® Series Reference Selector tool provides the ADC users an easy tool to identify the appropriate series voltage reference for his circuit. By selecting a Texas Instruments ADC from a list of 450+ Precision ADCs, a list of possible voltage reference devices along with a reference specification table is provided. The user can select their reference device or modify the design per their requirements.
- End of Manual -
Glossary of main parameters and units:
AVDD: ADC and Reference positive supply voltage
ADC: Analog-to-Digital Converter
ADC Resolution: The resolution of the converter indicates the number of discrete bits available on the device's output pin(s).
LSB size: least significant bit of the ADC is the ratio of Reference Voltage to the resolution of the ADC → LSB = Vref / 2N where Vref is the output voltage of the voltage reference device and N is the number of ADC bits.
SAR: Successive-approximation-register (SAR) analog-to-digital converters (ADCs) represent the majority of the ADC market for medium- to high-resolution ADCs.
Temperature range: each device has its specific operating temp range. The system's permissible operating temp range is usually determined by the device with the narrowest temp range (assuming that all parts on an electronic board are exposed to the same thermal conditions).
Vref: The output voltage of a voltage reference device. The term Vref is also used as a standard abbreviation for a Voltage reference device.
- "How voltage references affect mixed-signal parts", Baker, EDN 8/26/10
- "How the voltage reference affects ADC performance, Part 1", Baker, Oljaca, Texas Instruments, slyt330, page 5
- "How the voltage reference affects ADC performance, Part 2", Oljaca, Baker, Texas Instruments, slyt337, page 13
- "How the voltage reference affects ADC performance, Part 3", Baker, Oljaca, Texas Instruments, slyt354, page 5
June 9, 2014