Datasheet AD707 (Analog Devices) - 6

AD707 OFFSET NULLING OPERATION WITH A GAIN OF 100
The input offset voltage of the AD707 is the lowest available in Demonstrating the outstanding dc precision of the AD707 in a bipolar op amp, but if additional nulling is required, the practical applications, Table I shows an error budget calculation circuit shown in Figure 19 offers a null range of 200 µV. For for the gain of –100 configuration shown in Figure 21. wider null capability, omit R1 and substitute a 20 kΩ potenti- ometer for R2.
Table I. Error Budget +VS Maximum Error Contribution R1 0.1µF 10k
Ω
Av = 100 (C Grade) OFFSET ADJUST Error Source (Full Scale: VOUT = 10 V, VIN = 100 mV) 7 R2
V
2k
Ω OS 15 µV/100 mV = 150 ppm
1 2
IOS (100 Ω)(1 nA)/100 mV = 1 ppm
8
Gain (2 kΩ Load) (100 V/8 × 106)100 mV = 13 ppm
AD707 6
Noise 0.35 µV/100 mV = 4 ppm
3 4 0.1µF
VOS Drift (0.1 V/°C)/100 mV = 1 ppm/°C = 168 ppm
–VS
+1 ppm/°C
OBSOLETE
Figure 19. External Offset Nulling and Power Supply Total Unadjusted Error Bypassing @ +25°C = 168 ppm > 12 Bits @ –55°C to +125°C = 268 ppm > 11 Bits
GAIN LINEARITY INTO A 1 k
Ω
LOAD
The gain and gain linearity of the AD707 are the highest With Offset Calibrated Out available among monolithic bipolar amplifiers. Unlike other dc @ +25°C = 17 ppm > 15 Bits precision amplifiers, the AD707 shows no degradation in gain or @ –55°C to +125°C = 117 ppm > 13 Bits gain linearity when driving loads in excess of 1 kΩ over a ± 10 V output swing. This means high gain accuracy is assured over the
10k
Ω output range. Figure 20 shows the gain of the AD707, OP07, and
+VS 0.1µF
the OP77 amplifiers when driving a 1 kΩ load.
100
Ω The AD707 will drive 10 mA of output current with no signifi-
7 V 2 IN
cant effect on its gain or linearity.
AD707 6 VOUT 3 4 0.1µF AD707 99
Ω
–VS
Figure 21. Gain of –100 Configuration
OP07
Although the initial offset voltage of the AD707 is very low, it is nonetheless the major contributor to system error. In cases requiring additional accuracy, the circuit shown in Figure 19 can be used to null out the initial offset voltage. This method
OP77
will also cancel the effects of input offset current error. With the
@ +25
°
C
offsets nulled, the AD707C will add less than 17 ppm of error.
CHANGE IN OFFSET VOLTAGE – 10µV/Div RLOAD = 1k
Ω This error budget assumes no error in the resistor ratio and no
–15 –10 –5 0 5 10 15 OUTPUT VOLTAGE – V
errors from power supply variation (the 120 dB minimum PSRR of the AD707C makes this a good assumption). The external Figure 20. Gain Linearity of the AD707 vs. resistors can cause gain error from mismatch and drift over Other DC Precision Op Amps temperature. –6– REV. B