EVALPRAHVOPAMP-1RZ

EVALPRAHVOPAMP-1RZ User Guide UG-670 One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Evaluating Universal Precision High-Voltage Op Amps in SOIC Packages FEATURES LOW-PASS FILTER Footprint for 8-pin SOIC with bottom thermal pad Locations for passive filter components Figure 1 is a typical representation of a first-order low-pass filter. This circuit has a 6 dB per octave roll-off after a closed loop −3 dB point defined by fC. Gain below this frequency is defined as the magnitude of R2 to R1. The circuit can be considered an ac integrator for frequencies well above fC; however, the time domain response is that of a single RC, rather than an integral. GENERAL DESCRIPTION The EVALPRAHVOPAMP-1RZ is an evaluation board that accommodates single op amps in SOIC packages. It provides the user with multiple choices and extensive flexibility for different applications circuits and configurations. This board is not intended to be used with high frequency components or high speed amplifiers; however, it provides the user with many combinations for various circuit types such as active filters, differential amplifiers, and external frequency compensation circuits. A few examples of application circuits are shown in this user guide. C3 R2 ACL = −(R2/R1); closed loop gain Choose R4 equal to the parallel combination between R2 and R1 in order to minimize errors due to bias currents. DIFFERENCE AMPLIFIER AND PERFORMANCE OPTIMIZATION Figure 2 is useful as a computational amplifier in making a differential-to-single-ended conversion or in rejecting a common-mode signal. The output voltage VOUT is comprised of two separate components: 60 GAIN (dB) fL = 1/(2π × R1 × C3); unity gain frequency Figure 2 shows an op amp configured as a difference amplifier. The difference amplifier is the complement of the summing amplifier, and allows the subtraction of two voltages or the cancellation of a signal common to both inputs. The circuit shown in R1 fC 40 fC = 1/(2π × R2 × C3); −3 dB frequency 1. 20 2. fL A component VOUT1 due to VIN1 acting alone (VIN2 shortcircuited to ground). A component VOUT2 due to VIN2 acting alone (VIN1 shortcircuited to ground). R2 0 R1 10 100 1k RELATIVE FREQUENCY (f) 10k VIN2 VOUT R3 R4 Figure 1. Simple Low-Pass Filter 12040-002 1 12040-001 –20 VIN1 R2/R1 = 100 Figure 2. Difference Amplifier The algebraic sum of these two components must be equal to VOUT. By applying the principles expressed in the output voltage VOUT components, and by letting R3 = R1 and R4 = R2, then: VOUT1 = VIN1 R2/R1 VOUT2 = −VIN2 R2/R1 VOUT = VOUT1 + VOUT2 = (VIN1 − VIN2) R2/R1 PLEASE SEE THE LAST PAGE FOR AN IMPORTANT WARNING AND LEGAL TERMS AND CONDITIONS. Rev. A | Page 1 of 4 UG-670 EVALPRAHVOPAMP-1RZ User Guide For this type of application, CMRR depends upon how tightly matched resistors are used. Poorly matched resistors result in a low value of CMRR. CURRENT-TO-VOLTAGE CONVERTER Current can be measured in two ways with an operational amplifier. Current can be converted to a voltage with a resistor and then amplified, or injected directly into a summing node. R2 To see how this works, consider a hypothetical source of error for resistor R7 (1 − error). Using the superposition principle and letting R4 = R2 and R7 = R6, the output voltage is as follows: Figure 3. Current-to-Voltage Converter Figure 3 is a typical representation of a current-to-voltage transducer. The input current is fed directly into the summing node and the amplifier output voltage changes to exactly the same current from the summing node through R2. The scale factor of this circuit is R2 volts per amp. The only conversion error in this circuit is IBIAS, which is summed algebraically with IIN1. VDD = VIN2 − VIN1 From this equation, ACM and ADM can be defined as follows: ACM = R2/(R2 – R1) × error EXTERNAL COMPENSATION TECHNIQUES As mentioned above, errors introduced by resistor mismatch can be a big drawback of discrete differential amplifiers, but there are different ways to optimize this circuit configuration: 2. The differential gain is directly related to the ratio R2/R1. Therefore, one way to optimize the performance of this circuit is to place the amplifier in a high gain configuration. When larger values for resistors R2 and R4 and smaller values for resistors R1 and R3 are selected, the higher the gain, the higher the CMRR. For example, when R2 = R4 = 10 kΩ, and R1 = R3 = 1 kΩ, and error = 0.1%, CMRR improves to greater than 80 dB. For high gain configuration, select amplifiers with very low IBIAS and very high gain (such as the ADA4661-2, ADA4610-2, and AD8667) to reduce errors. Select resistors that have much tighter tolerance and accuracy. The more closely they are matched, the better the CMRR. For example, if a CMRR of 90 dB is needed, match resistors to approximately 0.02%. Series Resistor Compensation The use of external compensation networks is required to optimize certain applications. Figure 4 is a typical representation of a series resistor compensation for stabilizing an op amp driving capacitive load. The stabilizing effect of the series resistor isolates the op amp output and the feedback network from the capacitive load. The required amount of series resistance depends on the part used, but values of 5 Ω to 50 Ω are usually sufficient to prevent local resonance. The disadvantages of this technique are a reduction in gain accuracy and extra distortion when driving nonlinear loads. R02 VOUT VIN CL RL Figure 4. Series Resistor Compensation RL = 10kΩ CL = 2nF GND TIME (10µs/DIV) Figure 5. Capacitive Load Drive Without Resistor Rev. A | Page 2 of 4 12040-005 These equations demonstrate that when there is no error in the resistor values, the ACM = 0 and the amplifier responds only to the differential voltage applied to its inputs. Under these conditions, the CMRR of the circuit is highly dependent on the CMRR of the amplifier selected for this job. 12040-004 ADM = R2/R1 × {1 − [(R1 + 2R2/R1 + R2) × error/2]} 1. VOUT R4 VOLTAGE (200mV/DIV) VOUT   R2  R1 + 2R2  error     R1 1 − R1 + R2  × 2       =   2 R    VD +  × error     R1 + R2   IIN1 12040-003 Difference amplifiers are commonly used in high-accuracy circuits to improve the common-mode rejection ratio (CMRR). EVALPRAHVOPAMP-1RZ User Guide UG-670 GND GND TIME (10µs/DIV) 12040-008 VOLTAGE (200mV/DIV) RL = 10kΩ CL = 2nF 12040-006 VOLTAGE (200mV/DIV) RL = 10kΩ CL = 2nF TIME (10µs/DIV) Figure 6. Capacitive Load Drive with Resistor Figure 8. Capacitive Load Drive Without Snubber Snubber Network RL = 10kΩ CL = 500pF RS = 100Ω CS = 1nF VOLTAGE (200mV/DIV) Another way to stabilize an op amp driving a capacitive load is with the use of a snubber, as shown in Figure 7. This method presents the significant advantage of not reducing the output swing because there is no isolation resistor in the signal path. Also, the use of the snubber does not degrade the gain accuracy or cause extra distortion when driving a nonlinear load. The exact RS and CS combinations can be determined experimentally. RL TIME (10µs/DIV) 12040-007 VIN CL CS 12040-009 VOUT RS Figure 9. Capacitive Load Drive with Snubber Figure 7. Snubber Network C3 P1 1 2 3 GND V+ HIGH VOLTAGE V– AGND DNI R2 C4 DNI DNI V+ + V– V1 JOHNSON142-0701-801 AGND DNI 7 DUT 2 –IN V+ OUT 6 3 +IN PAD V– NC RT1 DNI G1 4 1 5 8 PAD AGND TP2 V2 JOHNSON142-0701-801 AGND TP0 G2 RO1 RO2 DNI DNI RS DNI CS DNI CL DNI AGND AGND ADA4700-1ARDZ R7 R3 1 2 3 45 AGND DNI C1 DNI 2 3 45 AGND C6 DNI R1 1 – R5 DNI C2 TP1 C5 DNI DNI C8 RT2 DNI R4 DNI AGND AGND DNI 1 VO JOHNSON142-0701-801 5 4 32 RL DNI G0 AGND AGND DNI AGND R6 DNI V– HIGH VOLTAGE – C7 + DNI AGND Figure 10. EVALPRAHVOPAMP-1RZ Electrical Schematic Rev. A | Page 3 of 4 12040-010 V+ EVALPRAHVOPAMP-1RZ User Guide 12040-011 UG-670 Figure 11. EVALPRAHVOPAMP-1RZ Board Layout Patterns REVISION HISTORY 5/14—Rev. 0 to Rev. A Changes to Figure 1 and Figure 2 ................................................... 1 Changes to Figure 3 .......................................................................... 2 Changes to Figure 7 and Figure 10 ................................................. 3 2/14—Revision 0: Initial Version ESD Caution ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality. 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