LM7705MMEVAL

User's Guide SNVA359B – October 2008 – Revised May 2013 AN-1883 LM7705 Evaluation Board 1 General Description This evaluation board is designed to demonstrate the functionality of Texas Instruments LM7705 Low Noise Negative Bias Generator. The board has a dual Operational Amplifier that can be connected to ground or biased with the on board LM7705. In the last case a True zero amplifier is demonstrated. 2 Basic Operation The LM7705 is a switched capacitor voltage inverter with a low noise, −0.23V fixed negative voltage regulator. This device is designed to be used with low voltage amplifiers to enable the amplifiers output to swing to zero volts, having a single supply. The -0.23 Volts is used to supply the negative supply pin of an amplifier. Rail-to-rail output amplifiers cannot output zero voltage when operating from a single supply voltage and can result in error accumulation due to amplifier output saturation voltage being amplified by following gain stages. A small negative supply voltage will prevent the amplifiers output from saturating at zero volts and will help maintain an accurate zero through a signal processing chain. This is illustrated in Figure 1. Additionally when an amplifier is used to drive an ADC's input, it can output a zero voltage signal and the full input range of an ADC can be used. The LM7705 has a shutdown pin to minimize standby power consumption. +V +V + In + - In -V CFLY CF+ VDD SHUTDOWN LM7705 CRES CRES VSS true zero output voltage CFVOUT SD low voltage amplifier -0.23V COUT VSS Figure 1. Typical LM7705 Application All trademarks are the property of their respective owners. SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated AN-1883 LM7705 Evaluation Board 1 Layout Considerations 3 www.ti.com Layout Considerations Careful consideration for circuitry design and PCB layout will eliminate problems and will optimize the performance of the LM7705. To ensure a clean supply voltage it is best to place decoupling capacitors close to the LM7705, between VDD and VSS. Another important issue is the value of the capacitors CFLY, CRES and COUT. In order to minimize the output voltage ripple low ESR capacitors are recommended. 4 LM7705 Evaluation Board Schematic Figure 2 shows the schematic of the evaluation board. C7 +5V 6.8 PF J1 TP5 U1 VIN 3 IN OUT C5 +5V 1 TP4 C1 C2 220 nF LM340AT-5.0 +5V 100 nF LMP7716 3 J2 100 nF AMP1 IN + 2 R5 GND 8 2 V+ 1 - 10k R1 TP2 10k U2A J3 AMP1 OUT R7 R2 10k 10k J5 TP6 GND R6 10k TP7 C9 opt R10 1 JP1 Shutdown CFLY+ CFLY- 8 7 CRES VSS LM7705 3 6 SD VOUT 4 VDD C13 C17 C11 C14 100 nF 10 PF 100 nF 10 PF VSS 5 C15 Opt TP1 JP2 C12 C16 C10 C6 22 PF opt 470 nF opt J4 V4 22 PF 2 +5V - R3 TP3 10k U2B 7 + 6 10k C18 U3 1 2 3 5 AMP2 IN 4.7 PF +5V LMP7716 J6 C4 +5V R9 R4 10k 10k C8 R8 6.8 PF 10k AMP2 OUT C3 1 2 3 100 nF Figure 2. Schematic of the LM7705 Evaluation Board 2 AN-1883 LM7705 Evaluation Board SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated LM7705 Evaluation Board Layout www.ti.com 5 LM7705 Evaluation Board Layout The layout of the evaluation board is given in Figure 3. Figure 3. Layout of the LM7705 Evaluation Board SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated AN-1883 LM7705 Evaluation Board 3 Bill of Materials 6 www.ti.com Bill of Materials Table 1 lists the Bill of Materials of the evaluation board. Table 1. Bill of Materials 4 Designator Description Comment R1, R2, R3, R4, R5, R6, R7, R8, R9, R10 0805 Resistor 10 kΩ TP1, TP2, TP3, TP4, TP5, TP7 testpoint white black TP6 testpoint JP1, JP2 3 PIN header J1, J5 Jack connector J2, J3, J4,J6 BNC staking .350 U1 LM340 TO-220 U2 LM7716 VSSOP-8 U3 LM7705 VSSOP-8 C1 0805 capacitor 220 nF C2, C3, C5, C11, C13 0805 capacitor 100 nF C4 0805 capacitor 4.7 uF C14, C17 0805 capacitor 10 uF C12, C18 0805 capacitor 22 uF C6, C9, C15, C16 Not Stuffed C7, C8 3216 capacitor 6.8 uF C10 0805 capacitor 470 nF AN-1883 LM7705 Evaluation Board SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Measurement Procedure www.ti.com 7 Measurement Procedure The performance improvement of an op amp (the device acts as a real True-zero amplifier, using the LM7705 for the biasing of the negative supply pin of the op amp), is shown in this section. The setup used for these measurements is given in Figure 4. A supply voltage between 8V to 16V is provided to connector J1. The ground connection is wired to J5. The on board 5V regulator provides a constant supply voltage for the op amps and the LM7705. The LM7705 can be activated by placing jumper JP1 in position 2-3. A signal generator is connected to J6 and generates a DC signal varying with a low frequency, with an amplitude of 25 mVPP, and an offset of 12.5 mV. Both amplifier U2A and U2B have a fixed gain setting of 2x. The output signal of the amplifier (J4) can be observed with a digitizing oscilloscope or an Analog to digital Converter board and processing software. The voltage level of the negative supply voltage pin of the op amps can be selected with jumper JP2. Placing JP2 in position 1-2, will provide a voltage of −0.23 to the negative supply voltage pin of the op amp. Placing JP2 in position 2-3 will connect the negative supply voltage pin of the op amp to ground. In this way only the output behavior of amplifier U2B is observed. POWER SUPPLY +V (J1) VOLTAGE SOURCE GND (J5) AMP 1 IN (J2) AMP 1 OUT (J3) AMP 2 IN (J6) AMP 2 OUT (J4) DIGITAL OSCILLOSCOPE LM7705 EVAL BOARD Figure 4. Measurement Setup SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated AN-1883 LM7705 Evaluation Board 5 Measurement Results 8 www.ti.com Measurement Results Figure 5 and Figure 6 shows the output voltage of op amp U2B without and with the LM7705. Output voltage measurement is performed with a 12 bit ADC. DIGITIZED OUTPUT VOLTAGE (V) DIGITIZED OUTPUT VOLTAGE (V) Figure 5 shows the output voltage of amplifier U2B, when the negative supply pin is connected to ground level. The voltage swing at the low side is limited by the saturation voltage of the amplifier. Figure 6 shows the output voltage swing of amplifier U2B when the negative supply pin is connected to the fixed negative output voltage of the LM7705. Using the LM7705 amplifier U2B can act as a True-zero amplifier. 0.050 0.040 0.030 0.020 0.010 0.000 VDSAT 0 80 160 240 320 400 0.050 0.040 0.030 0.020 0.010 0.000 0 80 6 AN-1883 LM7705 Evaluation Board 240 320 400 TIME (SAMPLES) TIME (SAMPLES) Figure 5. Amplifier Output Voltage without LM7705 160 Figure 6. Amplifier Output Voltage with LM7705 SNVA359B – October 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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