APPLICATION NOTE TDA8790M EVALUATION BOARD DOCUMENTATION AN/96031 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 APPLICATION NOTE TDA8790M EVALUATION BOARD DOCUMENTATION Author : Patrick LEJOLY Keywords: Evaluation board TDA8790M 8 bit ADC Low power ADC high speed ADC Date : April 1996 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 Summary This note describes a demonstration board which facilitates the evaluation of the TDA8790M 8 bit analog to digital converter. In addition the operation of the TDA8790M is shortly described and several methods to provide input offset , clamp, and top and bottom references are shown. 3 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 TABLE OF CONTENTS 1. INTRODUCTION 6 2. BOARD DESCRIPTION 8 2.1 CONNECTORS, POTENTIOMETERS AND SWITCHES POSITIONS 8 2.2 CONNECTORS, POTENTIOMETERS AND SWITCHES LIST 9 2.3 CONNECTORS, POTENTIOMETERS AND SWITCHES ROLE 11 3. EXAMPLES OF BOARD SETTINGS 12 3.1 INITIAL SETTINGS : 3.3 V ADC SUPPLIES, AC COUPLED INPUT, 40 MHz CLOCK (ON BOARD), 3.3 V and ≈ 1.2 V VOLTAGE REFERENCES 12 3.2 OTHER SETTINGS EXAMPLE : 5 V ADC SUPPLIES, AC COUPLED INPUT, 40 MHz CLOCK (ON BOARD), 3.3 V and ≈ 1.2 V VOLTAGE REFERENCES 14 4. A/D CONVERTER SUPPLIES 15 4.1 ON-BOARD SUPPLIES GENERATION 15 4.2 ON-BOARD / EXTERNAL SUPPLY SELECTION 16 5. VOLTAGE REFERENCES 17 5.1 TOP AND BOTTOM REFERENCES DERIVED FROM A POWER SUPPLY 19 5.2 TOP AND BOTTOM REFERENCES DERIVED FROM REFERENCE VOLTAGE REGULATOR(S) 20 5.3 ON-BOARD PRACTICAL REFERENCE VOLTAGES GENERATION 23 6. INPUT OFFSET 24 4 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 6.1 INPUT OFFSET DERIVED FROM A RESISTOR BRIDGE 24 6.2 INPUT OFFSET PRACTICAL GENERATION ON BOARD 25 6.3 INPUT OFFSET DERIVED FROM THE MEDIUM REFERENCE 25 6.4 INPUT OFFSET PROVIDED BY AN OPERATIONAL AMPLIFIER 27 7. CLAMP FOR VIDEO SIGNALS 29 8. CLOCK 31 8.1 CLOCK INPUT 31 8.2 ACTUAL IMPLEMENTATION ON THE EVALUATION BOARD 32 8.3 CLOCK JITTER 33 9. 8 BIT D/A CONVERTER 33 10. DEMO BOARD DOCUMENTATION : ELECTRICAL DIAGRAM, COMPONENT LIST & COMPONENTS IMPLANTATION 34 10.1 ELECTRICAL DIAGRAM 34 10.2 COMPONENT LIST 36 10.3 COMPONENTS IMPLANTATION 39 5 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 1. INTRODUCTION The TDA8790M is a 8 bit high speed, low voltage, low power analog to digital converter (30 mW typical at 3.3 V, 4 mW in standby mode). It has been designed for video signal digitizing, radio communication, camcorders & all applications where size and power saving are strong requirements. The supplies can be set in the range of 2.7 V up to 5.5V (down to 2.5 V for output supply). The sampling frequency can reach 40 MHz. Application requires few external components. TDA8790M comes in a plastic thin quad flat package SSOP20 (SOT266-1), with the following overall body dimensions: 6.5x4.4x1.3 mm3. The evaluation board described in this note was designed to allow a quick evaluation of the main TDA8790 characteristics. It is realized with a two layer PCB. The following features are included : - One single power supply (+8V +/-10%) is required to generate the supplies needed by the on-board ICs. Connection via banana plugs or grips is possible. To avoid supply connection errors, a green LED is on when the supply polarity is respected. A protection diode is also included to avoid any damage if an error occurs. - A high-speed 8-bit D/A converter TDA8702T for reconstrucion of the analog signal from the digital data output by TDA8790M. Because this D/A converter is not perfect, its analog output should not be used to characterize the TDA8790M. - A 40 MHz on-board quartz oscillator or an external clock can be used to control the TDA8790M. Additionally, the clock of the TDA8702T D/A converter can be either the same clock as the one used for TDA8790M (external clock or on board oscillator) or an independent external clock. - A 7805 regulator generates the 5V supply voltages needed by the D/A converter and the on-board clock oscillator. - A Texas Instruments TL431 reference supplies the supply voltage (switchable between +3.3V and +5V) and reference voltages for the TDA8790M. It is also possible to input directly the VDD supply voltage of the A/D converter through grip point TM3. 6 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 - Selection between on-board generated and external VRT (to be input on grip point TM1) A/D converter top reference voltage. - Probe connectors (CB1 for TDA8790M output bits and test points TP1, TP2 for the A/D converter clock signal and its ground) allow to connect a digital analyzer to the board and to perform digital treatment on the data output by TDA8790M. - AC input signals only are allowed. The input offset is provided by a resistor bridge. - All decoupling components necessary for good operation of the TDA8790M are included on board. 7 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 2. BOARD DESCRIPTION 2.1 CONNECTORS, POTENTIOMETERS AND SWITCHES POSITIONS K1 B1 B2 PHILIPS TM3 TM1 D3 TM2 K2 ALPAR PCB No 367 J2 K6 J3 K5 TP1 TP2 TP3 TDA8790 J1 P1 K4 D7 D6 D5 D4 D3 D2 D1 D0 8 bit connector & solder points P2 K3 TP4 K7 K8 CB1 TM5 TM4 J4 TDA8790M DEMO 8 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 2.2 CONNECTORS, POTENTIOMETERS AND SWITCHES LIST Reference B1 B2 CB1 Type Banana PLUG Banana PLUG test point array D3 J1 J2 J3 J4 K1 LED BNC BNC BNC BNC switch K2 switch K3 switch K4 switch K5 K6 switch switch Function External board supply (8V) External board supply (ground) TDA8790M output data D7 to D0 with respective grounds. Green LED, is ON when the power supply is on Signal input (50 Ω input) ADC (or ADC+DAC) clock input (50 Ω input) DAC clock input (50 Ω input) DAC output A/D converter power supply selection (between onboard generated +5V and on-board generated +3.3V). External/On-board generated A/D converter power supply selction. When external power supply is chosen, the supply voltage must be applied on grip point TM5. The +8V power supply (J13, J14) is still needed for the clock oscillator and D/A converter power supplies generation. Selection of the A/D converter top reference voltage VRT between external and on-board generated voltages. A/D converter on-board top reference selection. The two options are : 1) VRT= VDDA (should be used when VDDA=3.3V) 2) VRT derivated from VDDA through potentiometer P3 (when VDDA=5V, default setting in this configuration is VRT=3.3V) A/D converter Stand-By mode selection. A/D converter clock selection. The 2 options are : 1) A/D converter clock = on-board generated 40 MHz clock 2) A/D converter clock = external clock 9 Philips Semiconductors TDA8790M Evaluation board documentation Reference K7 Type switch K8 switch P1 potentiometer P2 potentiometer TM1 TM2 TM3 TM4 TM5 TP1 TP2 TP3 TP4 grip point grip point grip point grip point grip point test point test point test point test point Application Note AN96031 Function D/A converter clock selection. The 2 options are : 1) D/A converter clock = A/D converter clock 2) D/A converter clock = external clock Selection of the D/A converter analog output signal polarity. 2K potentiometer used to set the DC offset of the A/D converter analog input signal 5K potentiometer used to set the A/D converter top reference voltage when this voltage is not set equal to VDDA via switch K24 External board supply (+8V) External board supply (ground) External A/D converter supply (2.7 to 5.5 V) External top reference voltage (VRT) input Analog ground (close to TM1) A/D converter clock signal. A/D converter clock ground. A/D converter bottom reference voltage. A/D converter top reference voltage. 10 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 2.3 CONNECTORS, POTENTIOMETERS AND SWITCHES ROLE +3.3V / +5V VDD PHILIPS External VDD in A/D converter external clock input On-board VDD/ Ext. VDD CK +8V Green LED GROUND ALPAR PCB No 367 On-board oscillator External clock CKDA D/A converter external clock input CLOCK GROUND On / Stand-by Vrb GND D7 D6 D5 D4 D3 D2 D1 D0 VRB Analog input Offset trim TDA8790M OFF VRT for VDD= +5V / +3.3V IN GND VRT trim VRT Vrt External VRT On-board VRT External VRT in DAC Ext. clock / Same as ADC OUT/OUTN Analog output Vrt OUT TDA8790M DEMO 11 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 3. EXAMPLES OF BOARD SETTINGS 3.1 INITIAL SETTINGS : 3.3 V ADC SUPPLIES, AC COUPLED INPUT, 40 MHz CLOCK (ON BOARD), 3.3 V and ≈ 1.2 V VOLTAGE REFERENCES The boards are delivered set in the following configuration : - On-board generated TDA8790M supply voltage VDD is selected. It is set to +3.3V. - On-board generated VRT reference voltage is used. It is set to VRT = VDDA (+3.3V in the present case). - On-board generated + 40 MHz clock signal is used for both A/D converter and D/A converter. - Potentiometer P1 is adjusted to provide a mid-scale code output when no analog signal is input, in the given supply (VDD) and top reference voltage (VRT) conditions. - Sleep mode (stand-by mode) is inactive. - TDA8702T D/A converter output is not inverted. The board supply must be set at 8 V. In this configuration the analog input signal source must be provided by an external generator which is connected to the board by the J1 connector (dynamic input impedance: 50 Ω). 12 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 VDD PHILIPS +8V GROUND ALPAR PCB No 367 CK CKDA CLOCK GROUND GND Vrb D7 D6 D5 D4 D3 D2 D1 D0 VRB Analog input Offset trim OFF TDA8790M IN VRT GND Vrt Analog output Vrt OUT TDA8790M DEMO SWITCHES POSITION FOR ON-BOARD GENERATED VDD=+3.3V ON-BOARD GENERATED VRT AND CLOCK SIGNALS FOR A/D CONVERTER AND D/A CONVERTER (40 MHZ CLOCK) ARE USED (INITIAL SETTINGS) 13 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 3.2 OTHER SETTINGS EXAMPLE : 5 V ADC SUPPLIES, AC COUPLED INPUT, 40 MHz CLOCK (ON BOARD), 3.3 V and ≈ 1.2 V VOLTAGE REFERENCES VDD PHILIPS +8V GROUND ALPAR PCB No 367 CK CKDA CLOCK GROUND GND Vrb VRB Analog input OFF Offset trim TDA8790M IN VRT trim GND VRT D7 D6 D5 D4 D3 D2 D1 D0 Vrt Analog output Vrt OUT TDA8790M DEMO SWITCHES POSITION FOR ON-BOARD GENERATED VDD=+5V ON-BOARD GENERATED VRT AND CLOCK SIGNALS FOR A/D CONVERTER AND D/A CONVERTER (40 MHZ CLOCK) ARE USED 14 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 4. A/D CONVERTER SUPPLIES The TDA8790M can work with all supply voltages in the range of 2.7 V to 5.5 V: analog supply (VDDA), digital supply (VDDD), output supply (VDDO). Furthermore, VDDO can be set as low as 2.5 V. The only restriction for the supplies is to respect the following conditions: -0.2V < VDDA-VDDD < 0.2V -0.2V < VDDD-VDDO < 3V -0.2V < VDDA-VDDO < 3V 4.1 ON-BOARD SUPPLIES GENERATION The A/D converter supplies (VDDA=VDDD=VDDO) are provided by an adjustable precision regulator (TL431, IC3). The IC3 regulator output voltages are adjustable by a resistor ratio (see figure). The formula which gives the regulated voltage related to the resistor ratio is : external supply output R1 K1 R2 R3 R4 TL431 Vout = 2.5*(1+(R2+R3)/R4) or Vout = 2.5*(1+R3/R4) depending on K1 position The power consumption of the TL431 is volontarily set higher than necessary in order to allow different A/D converter voltage supplies. On board generated A/D converter supplies can be switched between 3.3 V and 5 V via swith K1. 15 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 4.2 ON-BOARD / EXTERNAL SUPPLY SELECTION It is also possible to choose between the voltage generated by the TL431 and an externally generated voltage for the TDA8790M A/D converter supply voltages (VDDA=VDDD=VDDO). The choice is made via switch K2. In the case the external supply voltage is chosen, it must be supplied through grip point TM3. It is also possible to replace resistors R1, R2, R3, R4 with values more suited to one’s peculiar purpose. Be sure to respect the difference limits between the suplies. In this case, the oscillator and D/A converter supply voltages are still generated by the 7805 regulator (IC4) which must still be supplied by the external +8V supply voltage. 16 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 5. VOLTAGE REFERENCES Here is a block diagram which explains the TDA8790 operation : Top reference (pin 10) Roffset I R E N C O D E R Middle reference output (pin 8) 8 R Bottom reference (pin 7) Roffset Analog signal input (pin 9) Comparators During the A to D conversion the analog input signal (pin9) is compared to voltage references by the means of voltage comparators (In fact these comparators are folding amplifiers). The full scale analog signal input range (FS) is given by: FS=8/9 (Top ref. - Bottom ref.) ; The 8/9 coefficient is due to the two offset resistors. The comparator voltage references are derived from a resistor ladder which is supplied through Vtop (pin 10) and Vbottom (pin 7). Therefore if the Vtop and Vbottom are not well regulated the A to D conversion will be affected. 17 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 Top reference (pin 10) is the highest voltage reference and the bottom reference (pin 7) is the lowest voltage reference. Consequently a current I is flowing from pin 10 to pin 7. The typical value for the internal resistor ladder is 2.1kΩ at 25°C. As shown in the following schematic the TDA8790M is versatile in the choice of the top voltage reference, bottom voltage reference and power supplies. So it will be easy to find top and bottom voltage references which fit with the majority of the applications. resistor ladder current: around 1 mA Offset resistor (top) Top reference (pin 10) Top reference range: 2.7V up to 5.5V; VCCA>=Top ref; VCCD>=Top ref Top offset= 1/18 * (Top - Bot) code 255 TDA8790 Internal resistor (2.1kΩ) Offset resistor (bottom) Top - Bottom range : 1.5V up to 2.7V (2.7V max.) code 0 Full scale input = 8/9 * (Top - Bot) Bottom offset = 1/18 * (Top - Bot) Bottom reference (pin 7) Bottom reference: ≥1.1V up to 3.75V 18 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 Regulation of Vtop & Vbottom depends on the level of cost and quality that are required by the customer application. Several methods providing these voltage references are shown in this section. 5.1 TOP AND BOTTOM REFERENCES DERIVED FROM A POWER SUPPLY If the power supply is well regulated a simple resistor string stucture will be efficient (see following figure). Power supply It is possible that the top reference equal the power supply voltage. In that case the Rtop resistor can be removed. An optional filter can be added on the analog supply (depending on supply noise level). Optionnal LC supply filter Rtop Top reference (pin 10) Typical voltages for a 3.3V application are 3.3V for the top reference and 1.2V for the bottom reference. The current flowing through the 2.1kΩ resistor ladder is ≈1mA. So Rtop=0 and Rbottom=1.2 kΩ. This is the solution implemented on the evaluation board. TDA8790 Internal resistor ladder (2.1kΩ) Medium reference (pin 11) Rbottom Bottom reference (pin 7) Remarks: here, the spreads due to process and temperature are not taken into account. Ground 19 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 5.2 TOP AND BOTTOM REFERENCES DERIVED FROM REFERENCE VOLTAGE REGULATOR(S) In some cases (noise on supply, several ADC’s mounted in parallel ...) solutions with precision regulators (Philips uA723, Texas TL431,...) may be preferred. TL431 output = 2.5 *(1+P1/R2) V 8V supply 3.3V R1=80Ω Top reference (pin 10) P1 TL431 R2=470Ω Ground TDA8790 Internal resistor ladder (2.1kΩ) Bottom reference (pin 7) P2 Ground 20 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 If several ADC are mounted in parallel, or if a very high precision of the voltage references over the whole temperature range is required, the following schematic can be used : Supply + supply Top reference (pin 10) R1 R2 TL431 R3 TDA8790 Internal resistor ladder (2.1kΩ) R4 R5 Bottom reference (pin 7) Ground + Ground Operational amplifiers with a low input offset should be used. The transistor types depend on the number of TDA8790M mounted in parallel. (1 mA typ. for one TDA8790M) If only one ADC is used the operational amplifier and the transistor which drive the top reference can be skipped. In this case, the voltage regulator directly drives the top reference. 21 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 In the following electrical diagram, top and bottom references are regulated by only one component (TL431): Supply R1 100nF Input signal (AC coupled) R3 Top reference (pin 10) Analog input (pin 9) TDA8790 Internal resistor ladder (2.1kΩ) R4 100nF TL431 Bottom reference (pin 7) R2 Ground The (top - bottom) difference is set at 2.5V by the TL431, so the full scale input is set at 8/9*2.5=2.22V. In addition the input offset is set at (Vtop+Vbottom)/2 by two equal resistors (R3 and R4). In this case the TL431 maintains the (top - bottom) difference at 2.5V over temperature and supply variations. Because the input offset is derived from the top and bottom references, it is also regulated at (Vtop+Vbottom)/2 over the temperature and supply variations. Using this method it is possible to drive the input offset and the top and bottom references of several TDA8790M with a very good matching with only one TL431. Typical resistor values for a 5V application and for one TDA8790M are: R3=R4=2.2 kΩ, R1=R2=150 Ω. The current flowing trough the R1, R2 resistors is around 8 mA (The TL431 requires a minimum current to provide a good regulation). 22 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 5.3 ON-BOARD PRACTICAL REFERENCE VOLTAGES GENERATION Pins VRT and VRB are decoupled to ground by 22nF ceramic capacitors. Power supply K4 P2 (5K) TM4 (for external VRT supply voltage connection) K3 TP4 Top reference (pin 10) TDA8790 Internal resistor ladder (2.1kΩ) Medium reference (pin 11) TP3 R6 (1K2) Bottom reference (pin 7) VRT is derived from VDD through potentiometer P2 = 5 kΩ. It is also possible to force VRT to VDD using switch K4 to short-circuit potentiometer P2 : Normally when VDD is set at +3.3V it is possible to short potentiometer P2 via switch K4 to set VRT to +3.3V = VDD. When VDD=5V, potentiometer P2 is used to set VRT to a voltage different from VDD (for example, as initially set on the board, +3.3V). Using K3 to disconnect potentiometer P2 from the A/D converter top reference input pin, it is possible to input the top reference voltage directly through grip point TM4 (and the associated ground TM5). Ground On the TDA8790M evaluation board, the VRB pin of the TDA8790M is connected to ground through a fixed 1.2 kΩ resistor (R6). The R6 resistor value has been chosen so that VRB be equal to +1.25V when VRT is equal to +3.3V. Thus for this board, whichever the TDA8790M voltage supply, the recommanded VRT voltage is VRT = +3.3V when VDDA >= +3.3V, and VRT = VDDA when VDDA < +3.3V. If necessary the R6 resistor can be replaced with another resistor of a different value for thorough evaluation of the A/D converter. However the VRB voltage should stay above +1.1V in any case. Test points TP3 and TP4 allow to measure respectively VRB and VRT voltages. 23 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 6. INPUT OFFSET When AC coupling is used with the TDA8790 it is necessary to provide an input offset in order to respect the TDA8790 full scale input range. Relations between the Vtop reference, the Vbottom reference, the maximum amplitude of the analog signal and the input offset are: Maximum amplitude of analog signal is (Vtop-Vbot)* 8/9 and the input signal is centered around the input offset which is (Vtop+Vbottom)/2. Consequently, if Vtop = 3.3 V and if Vbottom = 1.2 V the maximum amplitude of the analog signal is 1.8 V and the input offset is 2.25V; code 0 is obtained for a 1.316V input, and code 255 is obtained for a 3.184 V input. Input offset can be provided by many different methods; several methods are explained in this section. 6.1 INPUT OFFSET DERIVED FROM A RESISTOR BRIDGE supply input connector R + R C TDA8790 input pin P When a resistor bridge is used to provide an offset the current flowing through the resistors must be at least 10 times greater than the signal current (TDA8790 analog input current is 0 to 9µA) in order to guarantee the stability of the input offset. Consequently the resistive value of this resistor string must be below 30 kΩ (with a 3 V supply). ground If the input signal generator used to test the TDA8790 requires a 50 Ω load, R must be set at 73.33 Ω and P at 157.14 Ω (Vtop=supplies=3.3 V,Vbottom=1.2 V), in order that the dynamic impedance (R & P in parallel) be 50 Ω. Remarks: - This method provides a correct input offset but the current flowing through the resistor bridge is high: 14mA (R=75Ω, P=157Ω and 3.3V supply). 24 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 6.2 INPUT OFFSET PRACTICAL GENERATION ON BOARD In order to reduce this current consumption another method is used on board but it requires at least one more component (one resistor) : The analog signal input offset is derived from the ADC analog supply through a resistor bridge (R1, R7 and potentiometer P1). It is adjustable thanks to P1. R11 is equal to the output load of the external signal generator. C16 allows ground connection between R2 and (R5+ P1)//R7 in dynamic mode. Typical values when a 50 Ω signal generator is used are: R11=50 Ω, R5=330 Ω, R7=2 kΩ P1=2 kΩ (set approximately to 1.7 kΩ) , then the current flowing through the resistor bridge is only 750 µA with a 3.3 V VDD. VDDA R5 330Ω input connector J1 R11 50Ω C16 1µF + P1 2kΩ TDA8790 input pin R7 2kΩ Ground - When it is possible, it is better to replace the potentiometers by fixed resistors. This will avoid possible distortion effects on the input signal due to the capacitive components of the potentiometers. - It can be difficult to obtain the exact output load and the exact input offset when they are made up of fixed resistors, because the accuracy of the resistors is limited. Consequently in some professional applications it is better to provide the correct load and the correct input offset by means of operational amplifiers. 6.3 INPUT OFFSET DERIVED FROM THE MEDIUM REFERENCE In this case the input pin is connected to the medium voltage reference (pin 8) by the means of a resistor (R1). The medium voltage reference must be well decoupled by a capacitor (C1). The input impedance of the AD converter is given by R1 in parallel with Zin. 25 Philips Semiconductors TDA8790M Evaluation board documentation ground C1 analog signal + Vmed ref R1 TDA8790 + input pin C2 Application Note AN96031 This method gives good results in the following domains : high common mode supply rejection (because both the voltage references and the input offset are derived from the same supply), very low noise level and low cost. R1*C1 product must be high enough in order to avoid a coupling between the input signal and the medium reference. (C1=4.7µF for example) The offset on the input pin is: Vmed - (4.5µA*R1) 26 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 6.4 INPUT OFFSET PROVIDED BY AN OPERATIONAL AMPLIFIER The middle output reference voltage and a low input offset operational amplifier can be used to provide an accurate input offset. Several methods can be used : 1°) Top reference (pin 10) Vmedium reference (pin8) Analog signal R R - TDA8790 Internal resistor ladder (2.1kΩ) Signal input (pin 9) + Bottom reference (pin 7) The R resistor in the op-amp loop compensates the offset due to the R resistor connected to Vmed. (R=1kΩ; C=1µF). 2°) Top reference (pin 10) + C R1 R2 Vmedium output reference (pin8) - TDA8790 Internal resistor ladder (2.1kΩ) Signal input (pin 9) Analog signal Bottom reference (pin 7) 27 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 The amplifier does not need a high bandwidth, but the time necessary to load the C capacitor at ‘power on’ depends on.the op.amp maximum output current. The input impedance is R2 // Zin. Zin is the A/D converter input impedance. 3°) Top reference (pin 10) 2K Vmedium reference (pin8) + 2K R TDA8790 Internal resistor ladder (2.1kΩ) Signal input (pin 9) Analog signal Bottom reference (pin 7) Medium reference is derived from top and bottom reference (input impedance is R // Zin). 28 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 7. CLAMP FOR VIDEO SIGNALS In case the TDA8790M is used for video signals digitization, a clamp circuit can prove useful. The following schematics shows the principle of such a clamp circuit (temperature variations are NOT compensated). 29 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 30 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 8. CLOCK 8.1 CLOCK INPUT On the demo-board several methods can be used (depending on switch positions) to provide the ADC clock (see sections 2,3). Precautions must be taken if the high clock level is higher than the VDDD level. In fact, the TDA8790 clock input (pin 1) is protected by diodes (see figure) : VDDD clock input pin 1 VDDD/2 VSSD If the high clock level is greater than VDDD+0.5 V, a current will flow between the clock input & VDDD through the protection diode. This will affect the proper operation of the A/D converter. Consequently, it is necessary to keep the high clock level below VDDD+0.5 V. Several methods can be used to limit high clock level : 1- Use of a 3 V logic device as a 5 to 3 V interface (Philips LVC logic family for example). 5V logic 74LVC logic 31 TDA8790 clock input Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 2- Use of a parallel resistor/diode network in series with 5 V output (solution used on the evaluation board). 5V logic TDA8790 clock input The resistor limits current and voltage supplied to the TDA8790. The diode allows a faster high to low transition (R=350Ω, D=BAS16/BAS32...). 3- In case of 5 V logic with an open drain output, use a pull-up resistor connected to the low voltage supply. VDDD 5V logic TDA8790 clock input 8.2 ACTUAL IMPLEMENTATION ON THE EVALUATION BOARD 40MHz oscillator K6 BAS16 TDA8790 clock input J2 680 50 The resistor limits current and voltage supplied to the TDA8790. The diode allows a faster high to low transition (R=680Ω, D=BAS16/BAS32...). Using switch K6 it is possible to choose between external (plug J2) and on-board (40 MHz oscillator, Q1) clock for the A/D converter TDA8790M. 32 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 8.3 CLOCK JITTER If the clock jitter and the slope of the analog input signal are high, sampling errors can appear. Example : The equation of a sinewave signal is s(t)=A/2 sin( 2 π F t), where A is the ADC full scale amplitude (A=255 LSB) and F is the sinewave frequency. The slope of this signal is given by: ds(t)/dt=A/2 2 π F cos(2 π F t) This slope is maximum when t=0 (input voltage level is around middle code 127/128): ds(0)/dt=A π F Volt/second. That means that the middle code is available at the ADC input only during: Tlsb=LSB/(A π F)=(255 π F )-1 second. If the full scale sinewave frequency is F = 10 MHz, then Tlsb = 124 ps Consequently the clock jitter must be lower than this value. If a 20 MHz full scale sinewave is being sampled the jitter must be lower than 62 ps. Remarks: If the sample clock frequency and the input signal frequency have the same jitter (or phase noise), the sampling error due to jitter can be avoided. Therefore it is not suitable to do precise dynamic measurements of the ADC characteristics with the on board quartz oscillator. (Except if the input signal frequency and the quartz oscillator frequency are correlated). 9. 8 BIT D/A CONVERTER An 8 bit 5 V supply/TTL input DAC TDA8702 (IC2) allows rough A/D converter evaluation with a scope or a spectrum analyzer. Analog output level is in the range of 3.6 to 5 V (with a high impedance load). It is possible to switch the reconstructed Voutput signal present on plug J4 between non inverted and inverted via switch K8. The D/A converter supplies are set to 5 V (by means of a 7805 regulator, IC4). 33 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 The D/A converter clock can be chosen, via switch K7, between A/D converter clock (selected between on-board and external via switch K6) and external clock (input via plug J3). 10. DEMO BOARD DOCUMENTATION : ELECTRICAL DIAGRAM, COMPONENT LIST & COMPONENTS IMPLANTATION 10.1 ELECTRICAL DIAGRAM (see next page) 34 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 35 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 10.2 COMPONENT LIST Reference Value Component B1 B2 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 CB1 D1 D2 D3 IC1 BANANA PLUG (+8V) BANANA PLUG (GROUND) C1206 SPRAGUE_293D_D SPRAGUE_293D_D SPRAGUE_293D_D C1206 C1206 C1206 C805 C805 C805 C805 C1206 C805 C1206 C1206 C1812 SPRAGUE_293D_D SPRAGUE_293D_D SPRAGUE_293D_D C1206 C1206 C1206 C1206 C1206 SPRAGUE_293D_D SPRAGUE_293D_D 8 BITS CONNECTOR BAS16 BYV27_50 HLMP1700 (GREEN) TDA8790M 100nF 22 µF 22 µF 22 µF 100 nF 100 nF 100 nF 22 nF 22 nF 22 nF 22 nF 22 nF 100 nF 100 nF 100 nF 1 µF 22 µF 22 µF 22 µF 100 nF 100 nF 100 nF 22 nF 22 nF 33 µF 22 µF 36 Philips Semiconductors TDA8790M Evaluation board documentation IC2 Reference Value IC3 IC4 J1 J2 J3 J4 K1 K2 K3 K4 K5 K6 K7 K8 L1 L2 L3 L4 L5 L6 P1 P2 Q1 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 TC1-TC8 TM1 Application Note AN96031 TDA8702T Component TL431 7805 BNC BNC BNC BNC SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH 12µH LQH4N 12µH LQH4N 12µH LQH4N 12µH LQH4N 12µH LQH4N 12µH LQH4N 2K 3224W 5K 3224W 40MHz X071009 100 RC01 680 RC01 330 RC01 1K RC01 330 RC01 1K2 RC01 1K RC01 680 RC01 50 RC01 50 RC01 50 RC01 SOLDER POINTS GRIP POINT (+8V) 37 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 TM2 TM3 Reference Value GRIP POINT (GROUND) GRIP POINT (A/D converter VDD) Component TM4 TM5 TP1-TP2 TP3 GRIP POINT (A/D converter top ref.) GRIP POINT (GROUND) TEST POINTS (A/D converter clock) TEST POINT (A/D converter bottom reference) TEST POINTS (A/D converter top reference) TP4 J14 38 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 10.3 COMPONENTS IMPLANTATION (See next page) 39 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 40 Philips Semiconductors TDA8790M Evaluation board documentation Application Note AN96031 41