General Description The MAX460 is a high speed, JFET input voltage follower similar and pin compatible to the LH0033, but with input specifications significantly improved over the older device. This device is a direct pin-for- pin replacement for the EL2005. The cascode input stage maintains a constant high input resistance over the full +10V input voltage range. The input loading can be characterized as a 1000GQ resistance in parallel with a 3pF capacitor to ground. In most practical applications this can be considered a negli- gible load. Applications Fast Sample/Hold Amplifiers High Source Impedance Accurate Buffering Flash A/D Input Buffering Video Distribution CRT Drive Coaxial Line Driver Typical Operating Circuit SULA AL/VI High Accuracy Fast Buffer Features Pin for Pin Second Source! Pin Compatible with LH0033 and EL2005 Low Input Current 50pA Low Offset Voltage 2mvV Low Offset Drift 25uV/C High Slew Rate 1500V/us Fast Rise & Fall Times 2.5ns High Input Resistance 1000GQ. Wide Bandwidth 140MHz _ SOS eoeeee Ordering Information PART TEMP. RANGE MAX460MGC -5C to +125C MAX4601GC ~25C to -85C 12 Lead TO-8 EL2005G 55C 10 +125C Ssd12 Lead TO-8 EL2005CG -25C to +85C 12 Lead TO-8 (Note: The EL2005G is equivalent to the MAX460MGC. and the EL2005CG is equivalent to the MAX460/GC. PACKAGE 12 Lead 10-8 Pin Configuration MAXIM MAX460 +10V QUTPUT +1009 5, INPUT High Speed High Voltage Attenuator (Detailed Circuit Diagram Figure 5) Top View PRESET OFFSET ADJUST NC MAXAXLWVI Maxim integrated Products 1 MAXIM is a registered trademark of Maxim Integrated Products. O9PXVNMAX460 High Accuracy Fast Buffer ABSOLUTE MAXIMUM RATINGS Supply Voltage (Vt -V~) Lo. ec e eee eee 40V Maximum Power Dissipation ............... 1.5W (See Graph) Maximum Junction Temperature .............. 0.00 ee +175C Input Voltage Range .......... eect eee Vg Continuous Output Current ........... 2.0.0. +100mA Peak Output Current ...........0.0...2000000.-02002. ~250mMA Operating Temperature Range MAX460MGC 0... cee cece ccc eee eee ee -55C to +125C MAX460IGC 2... cece ene ~25C to +85C Storage Temperature Range ................. -65C to +150C Lead Temperature (Soldering, 10 sec.) ..............5. +300C Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (Vs = 15V, Vin = OV, Tain < Ta = Tmax) (Note 1) MAX460MGC MAX460IGC PARAMETER SYMBOL CONDITIONS UNITS MIN TYP MAX MIN TYP MAX Rg = 1000, Ty = +25C 2 5 3 10 Output Offset Vos s J mV Voltage Rg = 1002 10 15 Offset Tempco AVos/AT | Rg = 1002 (Note 3) 25 25 HVC Supply Rejection PSRR 410V < Vg = 20V 66 75 60 75 dB Ty = +25C (Notes 2 and 7) 2 50 5 100 pA Input bias current lp Ta = +25C, (Notes 4 and 7) 50 500 100 1000 Ty = Ta = Tax 2 5 0.6 5 nA . Ry = 1k. 0.97 0.98 1 0.96 0.98 1 . Voltage Gain Ay WV R, = 1000 0.92 0.95 0.98 0.91 0.95 0.99 -10V to +10V 2 1000 2 1000 Input Impedance Rin GQ Ty = 425C 10 1000 10 1000 Output Resistance Rout Vin = 21V 4 8 4 8 o Vin = 214, RL = 1kQ 12 12.5 12 12.5 Output Voltage Swing | Voyt Vin = 10.5V, R, = 1000, 9 og 9 98 Vv Ta = +25C . : External Offset Vos = OmV, ) Resistance Rext Ta = +25C (Note 6) s 200 7 200 e Supply Current Ig Vin = OV (Note 5) 19 22 19 24 mA Power Consumption Pg Vin = OV 570 660 570 720 mW AC ELECTRICAL CHARACTERISTICS (T, = +25C, Vg = +15V, Rg = 500, Ry = 1k) | MAX460MGC ~~ MAX4601GC PARAMETER SYMBOL CONDITIONS UNIT MIN TYP MAX MIN TYP MAX Slew Rate SR Vin = 10V, Vout = 25V 1000 1500 1000 1500 Ws Bandwidth BW Vin = 1Vams 140 140 MHz Phase Non-Linearity BW = 1 to 20MHz 2 2 deg. Rise Time t, Vy = 0.5V 25 2.5 ns Fall Time t AViy = 0.5V 1 1 ns Distortion HD F = 1kHz <04 < 0.1 % . Rg = 1000, Vin = 1Vams: Voltage Gain Ay E = tkHz 0.97 0.99 1 0.96 0.99 1 WV Output Resistance Rout Vin= 1Vams F = 1kHz 4 8 4 8 Qo Note 1: The MAX460MGC is 100% tested at +25C, +125C and -55C. The MAX460IGC is 100% production tested at +25C only. Specifications at temperature extremes are verified by sample testing to 10% LTPD, but these limits are not used to calculate outgoing quality level. Note 2: Specification is at +25C junction temperature due to requirements of high speed automatic testing. Note 3: Temperature coefficient measured from +25C to Tyax. Note 4: Measured in still air 7 minutes after application of power. Guaranteed through correlated automatic pulse testing. Note 5: Guaranteed through correlated automatic pulse testing at T, = +25C. Note 6: Offset adjust resistor connects between device pin 7 and V-. Note 7: Input bias current is guaranteed for -10V <= Viy = +10V. 2 MAAIMNICircuit Description The MAX460 combines a cascode JFET input stage with a high current bipolar output stage to form an analog buffer amplifier with very high input impedance and very low output resistance over a wide range of conditions. In normal operation, the source of Q1 wil be offset from the input voltage by the Vgs of Q1. The output is offset from the Q1 source voltage by the IR drop across R1 and the Vbe of Q5. The total of these offsets has been actively trimmed during the assembly process to be nearly zero. Q8 sets the drain to source voltage of Q1 to a low voltage that is virtually independent of input voltage, so that the input bias current only changes slightly when the input voltage is changed. (This is the primary difference between the MAX460 and the LH0033.) Q4, Q7 and Q9 are devices similar to Q1, Q5 and Q8. The current forced by the drain of Q9 will have the proper temperature coefficient to balance the tem- perature coefficient of the main amplifier stage, Q1. Diode connected transistors Q2 and Q3 provide a two Vbe voltage difference between the bases of the two output transistors, setting the quiescent current through Q5 and Q6. Resistors R3 and R4 provide a small amount of degeneration to stabilize the quies- cent current over temperature. RE 5 500 INPUT OVA OFFSET ADJUST R2 500) 6 q >a A OFFSET PRESET V- Figure 7. High Accuracy Fast Buffer Applications Layout Precautions The MAX460 should be treated as a high frequency amplifier when designing a printed circuit layout. Power supply bypassing to a ground plane with low inductance capacitors should be within a half inch of the device. For applications where the input capaci- tance is critical, connect the case of the device to the output so that the case capacitance is bootstrapped. For most applications, the case may be left uncon- nected or grounded. There is no internal connection to the case. In addition to the high frequency concerns, one must consider the effects of any possible leakage paths if the full input resistance of the MAX460 is to be util- ized. Ordinary printed circuit board materials may need a coating to prevent board leakage at high humidities or when dirty. The input in some situations may not even go to the printed circuit board, but instead be connected directly to a sensor or input connector. Lastly, consider the possibility of a guard structure surrounding the input node connected to the MAX460 output: since there will be little or no voltage differential between the input and output, there can be little input current flow even if there is some parasitic leakage resistance. Offset Voltage Adjustment For most normal applications of the MAX460, con- nect pin 6 to pin 7 and use the internally adjusted and guaranteed offset adjustment. When this is not acceptable, or there is a system offset to be ab- sorbed, an external 200 ohm trim pot may be con- nected from pin 7 to V-. Power Dissipation Considerations The MAX460 package is rated for 0.5W in still air at 125C and 0.75W with an infinite heat sink, Since the quiescent power is in the neighborhood of 600mW, a heat sink is needed for most 125C applications and some heavy load applications at lower temperatures. Note that several degrees rise in device temperature can have an adverse effect on the input current and resistance. Several suitable commercial heat sinks are available including the Thermalloy 2241, the Wake- field 215CB and the IERC UP-TO8-48CB. Please note that the can diameter is 0.55 inches nominal as opposed to the JEDEC TO-8 can which is 0.45 inches nominal. (See the outline drawing for detailed dimensions.) MAA O9PXVINNMAX460 High Accuracy Fast Buffer Operation from Single or Asymmetrical Power Supplies Since the MAX460 has no ground pin, an asym- metrical power supply is indistinguishable from a symmetrical supply with a DC level on the input. The single supply case is simply the asymmetrical case taken to the extreme of one of the supplies being zero. In either case, an offset error will be generated corresponding directly to the gain of the circuit times the apparent DC level with respect to a pseudo ground point halfway between the supplies. Output Offset = 0.5(1 - gain)(V*] -|V74) For example, a device operating on supplies of +5V and -12V would have an apparent offset error due to the gain of about -35mV. This could easily be cor- rected with an offset adjust pot connected from pin 7 to V" as discussed in the offset voltage adjustment section. Capacitive Loading The MAX460 is designed to drive heavy capacitive loads without susceptibility to oscillation. Note that the absolute maximum current rating must still be observed, thus the output slew rate times the load capacitance must be less than 250mA. For example, a 1000V/us slew rate with a 250pF load would fall just within the absolute maximum peak current specifica- tion. If a heavier capacitive load needs to be driven, the slew rate must be externally limited. Power dissi- pation resulting from capacitive load currents must be considered independently. The real power dissi- pated in a circuit driving a sine wave into a pure capacitive load is: Pac = (Vp-p) * Frequency C, This dissipation adds directly to the devices quies- cent power and any DC load power that might be present. The sum of all these terms must be less than the absolute maximum power rating at the tempera- ture of operation. For example, a 250pF load driven to 20V peak to peak at 1MHz adds a reactive power dissipation in the MAX460 of: (20)2 x 108 x 250 x 10-12 = 100mW. This additional power is not often a severe appli- cation problem with the MAX460. Short Circuit Protection The MAX460 is not internally short circuit protected as most of the possiblities involve some compro- mise in output swing or transient response. The output stage collectors are available separately, so there are several options open to the user. The simplest and most commonly used is the simple resistor in each output stage collector. For worst case protection these resistors may be calculated by: Ruim = V+/100mA = V-/100mA = 1502 for 15V supplies Unfortunately, a resistor this large severely restricts the voltage swing into a heavy load and the slew rate into a capacitive load. Decoupling the Vo* and Vo pins with capacitors will retain full output swing for transient pulses, but if the capacitors are made too large (to hold up long pulses) the protection is lost. A better but more complex circuit is shown in figure 4. Here, each output stage collector is driven by a current source set to a safe current, in this case, about 70mA. Ordinarily, the actual output current demand is less than that, so the current source saturates, applies +V, and -V, to the output collectors and the MAX460 behaves normally. In the event of a short on the output, however, the current source comes into play and reduces the output stage collector voltage as required to keep the current to a safe level. The output stage collectors may be bypassed with a small capacitor to give additional current capacity for short periods, as would be required in driving a capacitive load. O+15V INPUT OFFSET PRESET (OPEN) 15 Figure 2. Offset Zero Adjust Figure 3. Using Resistor Current Limiting 4 Q1 = 03 = 2N2005 A2 = A4 = 2N2219 Yr Rum 102 Figure 4. Current Limiting Using Current Sources 15 MAXLWVIHigh Accuracy Fast Buffer Typical Applications High Resistance Compensated Divider to Monitor +100V The circuit in Figure 5 is intended to interface an A/D converter with a maximum input voltage of +10V to an input signal of +100V with a minimum of loading on the signal. Resistors R1 and R2 and capacitor C1 form a frequency compensated 10:1 voltage divider +loov so that the buffer never sees more than its rated INPUT +10V. For optimum transient response, C1 should be adjusted to compensate for variations in stray capacitance. Note that this circuit will work with the LH0033 device, but there will be a tendency for the negative gain to be in error due to the rise in the input current for - a _ negative input voltages. (See the curve of input bias current vs. input voltage on the LH0033 data sheet.) This non-linearity at the input can cause apparent offset voltage changes in response to an AC signal that would cause the input current to be higher at Figure 5. High Speed High Voltage Attenuator some parts of the cycle than at others. Typical Operating Characteristics (INPUT BIAS CURRENT vs INPUT BIAS CURRENT vs INPUT BIAS CURRENT INPUT VOLTAGE TEMPERATURE DURING WARM-UP 10 10 1000 1 t A z = = = 5 | = # 0 2 a = 5 3 ~~ 3 2 2 A og 100 = 0.01 5 001 = = = = 0.001 0.001 0.0001 0.0001 10 Ni 0 15 0 2 15 125 q z 4 6 8 10 INPUT VOLTAGE [) TEMPERATURE (C) TIME FROM POWER TURN-ON (MINUTES] RISE TIME vs TEMPERATURE SMALL SIGNAL PULSE RESPONSE LARGE SIGNAL PULSE RESPONSE 5 +20 Vs + +15 Ru kD 4 +10 z | = = z ? 2 5 ro : 2" = 2 A = = 10 1 0 -20 -50 25 75 125 0 2 4 6 8 10 Q 1a 20 30 40 50 TEMPERATURE (C} TIME (ns] TIME {ns} MAXIM 5 O9PXVNMAX460 | High Accuracy Fast Buffer MAXIMUM POWER DISSIPATION GAIN vs INPUT VOLTAGE Typical Operating Characteristics OUTPUT RESISTANCE vs OUTPUT CURRENT 80 a 80 160 OUTPUT CURRENT (mA} OFFSET VOLTAGE vs SUPPLY VOLTAGE Ry = kt) Th 5 20 SUPPLY VOLTAGE (+V) 25 1.00 CASE 20 = = = = z # = 15 IN = z = = = = > 5 PSK] N = 00s 2 = ig [AMBUENT N 3 = as" 4K = Gr Z ~s E 05 0 0.90 0 2 5 7 100 125 150 5-100 -5 0 5 WwW 15 -160 TEMPERATURE (C} INPUT VOLTAGE (V] SUPPLY CURRENT vs FREQUENCY RESPONSE SUPPLY VOLTAGE 24 40 RL = 1k. \ 2 30 z = La = z= ao = = BH 1 we 20 = eo Ln = z B= [Le 3 3 16 a 10 a @ NX & 14 0 MN 12 -10 2M 20M 200M 5 10 15 20 5 FREQUENCY (Hz) SUPPLY VOLTAGE (+ V) 0.595 - 0.605 (is113 ream ean TYP. 0.100 TYP. 0.545 - (556 (2540) (agas 14.097) 2 4 ean v 0.022 - 0.030 L n.148- 0.181 bee OSD (2759 4507) (0.559 - 0.762) 4 e008 45 _ 0.026 - 0.036 4 esco-o91g DIED 0400 typ _ 0.500 - 0.560_ (10.160) (12.700 - 14.224} 2 0.026 - 0.036 O.016- 0.019 yi, 0.060 , (500 = 0.914 (0.406 0.483] [152g DA TYP MAXAIL/VI