Datasheet AD549 (Analog Devices) - 10

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AD549 Data Sheet FUNCTIONAL DESCRIPTION MINIMIZING INPUT CURRENT CIRCUIT BOARD NOTES
The AD549 is optimized for low input current and offset A number of physical phenomena generate spurious currents voltage. Careful attention to how the amplifier is used reduces that degrade the accuracy of low current measurements. Figure 27 input currents in actual applications. is a schematic of a current to voltage (I-to-V) converter with Keep the amplifier operating temperature as low as possible to these parasitic currents modeled. minimize input current. Like other JFET input amplifiers, the
CF
AD549 input current is sensitive to chip temperature, rising by a factor of 2.3 for every 10°C. Figure 25 is a plot of the AD549
RF 2
input current vs. ambient temperature.
AD549 6 + 1nA VOUT fS 3 8 – 100pA NT 10pA V dCP dV R + C P CP II' = + V P RP dT dT
7 02
CURRE 1pA
1-
V
51
S AS
00
BI
Figure 27. Sources of Parasitic Leakage Currents
UT 100fA INP
Finite resistance from input lines to voltages on the board, modeled by Resistor R
10fA
P, results in parasitic leakage. Insulation 5 resistance of more than 1015 Ω must be maintained between -02 1 51 the amplifier signal and supply lines to capitalize on the low
1fA
00
–55 –25 5 35 65 95 125
input currents of the AD549. Standard PCB material does not
TEMPERATURE (°C)
have high enough insulation resistance; therefore, connect the Figure 25. Input Bias Current vs. Ambient Temperature input leads of the AD549 to standoffs made of insulating On-chip power dissipation raises the chip operating tempera- material with adequate volume resistivity (that is, Teflon®). The ture, causing an increase in input bias current. Due to the low surface of the insulator must be kept clean to preserve surface quiescent supply current of the AD549, the chip temperature resistivity. For Teflon, an effective cleaning procedure consists is less than 3°C higher than its ambient temperature when the of swabbing the surface with high grade isopropyl alcohol, (unloaded) amplifier is operating with 15 V supplies. The rinsing with deionized water, and baking the board at 80°C for difference in the input current is negligible. 10 minutes. However, heavy output loads can cause a significant increase in In addition to high volume and surface resistivity, other proper- chip temperature and a corresponding increase in the input ties are desirable in the insulating material chosen. Resistance current. Maintaining a minimum load resistance of 10 Ω is to water absorption is important because surface water films recommended. Input current vs. additional power dissipation drastically reduce surface resistivity. The insulator chosen due to output drive current is plotted in Figure 26. should also exhibit minimal piezoelectric effects (charge
6
emission due to mechanical stress) and triboelectric effects (charge generated by friction). Charge imbalances generated
NT
by these mechanisms can appear as parasitic leakage currents.
5
These effects are modeled by Variable Capacitor CP in Figure 27.
CURRE
Table 3 lists various insulators and their properties.2
AS 4 BI
Guarding the input lines by completely surrounding them with
BASED ON UT TYPICAL IB = 40fA
a metal conductor biased near the potential of the input lines
NP 3 D I
has two major benefits. First, parasitic leakage from the signal
E IZ
line is reduced because the voltage between the input line and
AL RM 2
the guard is very low. Second, stray capacitance at the input
NO
6 node is minimized. Input capacitance can substantially degrade -02 1 51 signal bandwidth and the stability of the I-to-V converter.
1
00
0 25 50 75 100 125 150 175 200 ADDITIONAL INTERNAL POWER DISSIPATION (mW)
2 Electronic Measurements, pp. 15–17, Keithley Instruments, Inc., Cleveland, Figure 26. Input Bias Current vs. Additional Power Dissipation Ohio, 1977. Rev. K | Page 10 of 18 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS FUNCTIONAL DESCRIPTION MINIMIZING INPUT CURRENT CIRCUIT BOARD NOTES OFFSET NULLING AC RESPONSE WITH HIGH VALUE SOURCE AND FEEDBACK RESISTANCE COMMON-MODE INPUT VOLTAGE OVERLOAD DIFFERENTIAL INPUT VOLTAGE OVERLOAD INPUT PROTECTION SAMPLE-AND-DIFFERENCE CIRCUIT TO MEASURE ELECTROMETER LEAKAGE CURRENTS PHOTODIODE INTERFACE Photodiode Preamp Noise LOG RATIO AMPLIFIER TEMPERATURE COMPENSATED pH PROBE AMPLIFIER OUTLINE DIMENSIONS ORDERING GUIDE