SBOS502F September   2009  – December 2016 REF5025-HT

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Output Adjustment Using the TRIM/NR Pin
      2. 7.3.2 Low Temperature Drift
      3. 7.3.3 Temperature Monitoring
      4. 7.3.4 Noise Performance
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Negative Reference Voltage
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Positive Reference Voltage
        1. 8.2.2.1 Detailed Design Procedure
          1. 8.2.2.1.1 Load Capacitance
          2. 8.2.2.1.2 Bandgap Noise Reduction
    3. 8.3 System Example
      1. 8.3.1 Data Acquisition
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Power Dissipation
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

8.1 Application Information

The REF5025-HT device is a low-noise, precision bandgap voltage reference that is specifically designed for excellent initial voltage accuracy and drift. See the Functional Block Diagram.

When designing circuits with a voltage reference, output noise is one of the main concerns. The main source of voltage noise in the reference voltages originates from the bandgap and output amplifier, which contribute significantly to the overall noise. During the design process, it is important to minimize these sources of voltage noise.

8.2 Typical Applications

8.2.1 Negative Reference Voltage

For applications requiring a negative and positive reference voltage, the REF5025-HT and OPA735 can provide a dual-supply reference from a 5-V supply. Figure 27 shows how the REF5025-HT provides a 2.5-V supply reference voltage. The low-drift performance of the REF5025-HT complements the low offset voltage and zero drift of the OPA735 to provide an accurate solution for split-supply applications. Take care to match the temperature coefficients of R1 and R2.

REF5025-HT ai_pos_neg_vref_bos471.gif Figure 27. The REF5025-HT and OPA735 Create Positive and Negative Reference Voltages

8.2.1.1 Design Requirements

When using REF5025-HT in the design, it is important to select a proper capacitive load that do not create gain peaking adding noise to the output voltage. At the same time, the capacitor must be selected to provide required filtering performance for the system. Input bypass capacitor and noise reduction capacitors must be added for optimum performances.

8.2.1.2 Detailed Design Procedure

Proper design procedure will require first to select output capacitor. If the ESR of the capacitor is not in 1-Ω range additional resistor must be added in series with the load capacitor. Next, add a 1-µF capacitor to the NR pin to reduce internal noise of the REF5025-HT. Measuring output noise will confirm if the design has met the initial target.

8.2.1.3 Application Curves

REF5025-HT app-curve-1.png Figure 28. Noise Measurements of Properly Design REF5025 Data Acquisition System From Figure 35
REF5025-HT freq_spectrum_02_bos471.png Figure 29. FFT of Data Acquisition System Design With REF5025 From Figure 35

8.2.2 Positive Reference Voltage

REF5025-HT REF5040_Load_Capacitance_SBOS471.gif Figure 30. REF5025-HT With Load Capacitor

8.2.2.1 Detailed Design Procedure

8.2.2.1.1 Load Capacitance

To determine how much noise the reference voltage is contributing in a real application, this design uses the circuit presented in Figure 30. For the same conditions as power supply, input decoupling, and load current, measure the output noise for different output decoupling or load capacitors. The load capacitor type will change the low-pass filter frequency that is created on the output. This filter is determined by an added capacitor value and two parasitic components: the open-loop output impedance of the internal amplifier to the reference voltage, and the ESR of the external capacitor.

Figure 31 shows a fast-Fourier-transform (FFT) plot of the output signal of the reference voltage circuit with a 10-μF ceramic capacitor load. The output noise level peaks at around 9 kHz because of the response of the internal amplifier of the circuit to the capacitive load (CL).

This peaking is the main contributor to the overall measured noise. This output noise, measured with an analog meter over a frequency range of up to 80 kHz, is approximately 16.5 μVRMS. If the voltage-reference circuit was connected to the input of an ADC, the measured noise across a 65-kHz frequency range would be 138 μVPP. This noise level makes this solution adequate for 8- to 14-bit converters.

REF5025-HT REF5040_Output_Capacitor_SBOS471.png Figure 31. REF5025 FFT Plot of the Noise
With 10-µF Load Capacitor and 10-µΩ ESR

Every capacitor can be represented with a complicated equivalent model, which is voltage and frequency dependent with a large number of passive components. For the purposes of this design, this model is limited to the few components. The biggest impact on the creation of the low-pass filter and stability analysis is the simplified model of equivalent series inductance and resistance. Considering good layout practice and inherently low equivalent series inductance of today’s components, this model in the future analysis will be presented only by equivalent capacitance and series resistance.

REF5025-HT REF5040_ESR_SBOS471.gif Figure 32. Equivalent SCH of REF5025 With Load Capacitor for Stability Analysis

When evaluating the impact of ESR and CL on the performance the reference voltage, it is important to include the effect of the open-loop output resistance (RO) of the output amplifier. The combination of RO, ESR, and CL modifies the open-loop response curve by introducing one pole (fP) and one zero (fZ). The values RO, ESR, and CL determine the corner frequency of the added pole fP; and the values of ESR and CL determine the corner frequency of the added zero.

The introduction of the external ESR-CL on the output of the reference voltage modifies the output amplifier open-loop gain curve. The added pole modifies the open-loop gain curve of the reference voltage output amplifier by introducing a –20 dB/decade change at the frequency fP to the already –20 dB/decade slope of the open-loop gain curve, making the slope equal to –40 dB/decade. The added zero at frequency fZ changes the open-loop gain curve back to –20 dB/decade.

Table 1. Noise Measurement Results for Different Load Capacitors

NOISE 22 kHz
LP-5P		 30 kHz
LP-3P 		80 kHz
LP-3P		 > 500 kHz UNIT
GND 0.8 1 1.8 4.9 µVRMS
1 µF 37.8 41.7 53.7 9017
2.2 µF (cer) 41.7 46.2 55.1 60.8
10 µF 33.4 33.4 35.2 38.5
10 µF (cer) 37.1 37.2 37.8 39.1
20 µF (cer) 33.1 33.1 33.2 34.5
47 µF 23.2 23.8 24.1 26.5

Table 1 shows the measured noise values for different frequency bandwidths as well as different values and types of external capacitors. These measurements show that low-ESR (approximately 100-mΩ) ceramic capacitors tend to increase the noise, compared to normal-ESR (approximately 2-Ω) tantalum capacitors. This tendency is caused by a stability issue with the output amplifier and gain peaking in the amplifier frequency response.

8.2.2.1.2 Bandgap Noise Reduction

REF5025-HT Bandgap_Noise_Reduction_bos471.gif Figure 33. REF5025-HT Internal Structure of Trim/NR Pin

The internal schematic of the REF5025-HT device shows that the trim pin allows direct access to the bandgap output. Figure 33 shows the trim pin connection to the internal bandgap circuit through a resistor. Adding a capacitor on the trim pin creates a lowpass filter that has a broadband attenuation of −21 dB.

For example, a small 1-μF capacitor adds a pole at 14.5 Hz and a zero at 160 Hz. If more filtering is needed, a larger value capacitor can be added, which will lower the filter cutoff frequency and the noise contributed by the bandgap.

Table 2. Measured Noise (µVRMS) for Four Bandwidths

NOISE 22 kHz (LOW-PASS 5-POLE) 30 kHz (LOW-PASS 3-POLE) 80 kHz (LOW-PASS 3-POLE) > 500 kHz UNIT
GND 0.8 1 1.8 4.6 µVRMS
2.2 µF (ceramic) 42.5 47.2 61.2 68.3
2.2 µF + 1 µF 17.5 19.4 22.6 24.5
10 µF (ceramic) 34.4 35.6 37.7 44.5
10 µF + 1 µF 14.1 14.4 14.9 16.4
20 µF (ceramic) 34.8 34.9 35.1 35.2
20 µF + 1 µF 14.4 14.4 14.7 15.1

Adding a 1-μF capacitor in this example filters the noise contribution of the bandgap and lowers the total noise by a factor of 2.5 times.

8.3 System Example

8.3.1 Data Acquisition

Data acquisition systems often require stable voltage references to maintain accuracy. The REF5025-HT family features low noise, very low drift, and high initial accuracy for high-performance data converters. Figure 34 shows the REF5040 as an example in a basic data acquisition system. The same principle can be applied when designing with REF5025-HT.

REF5025-HT ai_basic_data_acq_bos410.gif Figure 34. Basic Data Acquisition System

During the design of the data acquisition system, equal consideration must be given to the buffering analog input signal as well as the reference voltage. Having a properly designed input buffer with an associated RC filter is a necessary requirement, but does not ensure the maximum performance.

REF5025-HT data_acquisition_system_bos471.gif Figure 35. Complete Data Acquisition System Using REF50xx

Three measurements using different components of the output are shown for this data acquisition system.

Table 3 shows improvements on the FFT for a properly designed system.

Table 3. Data Acquisition Measurement Results for Different Conditions

OPA365
REF5040
TRIM		 124 Ω, 1 nF
10 µF
0 µF		 124 Ω, 1 nF
10 µF + 47 µF
1 µF		 124 Ω, 100 µF
10 µF + 47 µF
1 µF		 UNIT
Resolution 16 16 16 Bits
States 65536 65536 65536
VREF 4.096 4.096 4.096 V
LSB 62.5 62.5 62.5 µV
VIN 4.02 4.02 4.02 V
Data Std 1.07 0.53 0.41 LSB
Noise 67.0 33.4 25.8 µVRMS
Noise 442.3 220.5 170.2 µVPP
SNR 86.7 92.8 95.0 dB
FTT Points 32768 32768 32768
Noise Flor –128.8 –134.9 –131.7 dB

Once the correct components for data acquisition system from Figure 35 are selected, measurement results can be compared to the ADS8326 data sheet specifications.

Table 4. AC Performance for Data Acquisition System From Figure 35

REF5040
TRIM		 ADS8326
DATA SHEET		 ADS8326B
DATA SHEET		 SYSTEM
LOW ESR		 SYSTEM
10 µF + 47 µF
1µF		 UNIT
SNR 91 91.5 90.6 92.2 dB
SINAD 87.5 88 85.7 89.5 dB
SFDR 94 95 88.3 98.4 dB
THD –90 –91 –87.3 –92.9 dB
ENOB 14.28 14.35 13.94 14.58 Bits