Datasheet AD625 (Analog Devices) - 9

AD625
Any resistors in series with the inputs of the AD625 will degrade
RTO NOISE RTO OFFSET VOLTAGE
the noise performance. For this reason the circuit in Figure 26b should be used if the gains are all greater than 5. For gains less
300 nV Hz
than 5, either the circuit in Figure 26a or in Figure 26c can be
– 3
used. The two 1.4 kΩ resistors in Figure 26a will degrade the
200
noise performance to:
2 100
4 kTR +(4 nV/ Hz)2 = 7.9 nV/ Hz
MULTIPLYING FACTOR
ext
VOLTAGE NOISE 10k 20k 30k 40k 50k 60k 10k 20k 30k 40k 50k 60k RESISTOR PROGRAMMABLE GAIN AMPLIFIER FEEDBACK RESISTANCE – FEEDBACK RESISTANCE –
In the resistor-programmed mode (Figure 27), only three exter-
RTO OFFSET VOLTAGE DRIFT BANDWIDTH
nal resistors are needed to select any gain from 1 to 10,000.
6 1M 10k
Depending on the application, discrete components or a
5
pretrimmed network can be used. The gain accuracy and gain
Hz – 100k 20k
TC are primarily determined by the external resistors since the
4
AD625C contributes less than 0.02% to gain error and under
50k 3
5 ppm/°C gain TC. The gain sense current is insensitive to
10k
common-mode voltage, making the CMRR of the resistor pro-
2 FREQUENCY MULTIPLYING FACTOR
grammed AD625 independent of the match of the two feedback
1
resistors, RF.
10k 20k 30k 40k 50k 60k 1 10 100 1k FEEDBACK RESISTANCE – FEEDBACK RESISTANCE – Selecting Resistor Values
As previously stated each R Figure 28. RTO Noise, Offset, Drift and Bandwidth vs. F provides feedback to the input stage and sets the unity gain transconductance. These feedback Feedback Resistance Normalized to 20 kΩ resistors are provided by the user. The AD625 is tested and specified with a value of 20 kΩ for R
Table I. Common Gains Nominally Within 0.5% Error
F. Since the magnitude of RTO errors increases with increasing feedback resistance, values
Using Standard 1% Resistors
much above 20 kΩ are not recommended (values below 10 kΩ
GAIN R
for R
F RG
F may lead to instability). Refer to the graph of RTO noise, offset, drift, and bandwidth (Figure 28) when selecting the 1 20 kΩ ∞ feedback resistors. The gain resistor (RG) is determined by the 2 19.6 kΩ 39.2 kΩ formula RG = 2 RF/(G – l). 5 20 kΩ 10 kΩ 10 20 kΩ 4.42 kΩ
2R G = +1 F
20 20 kΩ 2.1 kΩ
RG
50 19.6 kΩ 806 Ω
RF RG RF +INPUT –INPUT
100 20 kΩ 402 Ω 200 20.5 kΩ 205 Ω
1 16 +GAIN –GAIN
500 19.6 kΩ 78.7 Ω
SENSE SENSE 2 15
1000 19.6 kΩ 39.2 Ω
RTI NULL RTO 3 14
4 20 kΩ 13.3 kΩ
NULL +VS RTO
8 19.6 kΩ 5.62 kΩ
4 13 RTI NULL NULL
16 20 kΩ 2.67 kΩ
A1 A2 5 12 +GAIN DRIVE –GAIN DRIVE
32 19.6 kΩ 1.27 kΩ
NC 6 11
64 20 kΩ 634 Ω
10k 10k V REF OUT 7 10
128 20 kΩ 316 Ω
10k 10k A3
256 19.6 kΩ 154 Ω
–V 8 9 +V S S AD625
512 19.6 kΩ 76.8 Ω 1024 19.6 kΩ 38.3 Ω Figure 27. AD625 in Fixed Gain Configuration A list of standard resistors which can be used to set some com-
SENSE TERMINAL
mon gains is shown in Table I. The sense terminal is the feedback point for the AD625 output For single gain applications, only one offset null adjust is neces- amplifier. Normally it is connected directly to the output. If sary; in these cases the RTI null should be used. heavy load currents are to be drawn through long leads, voltage drops through lead resistance can cause errors. In these in- stances the sense terminal can be wired to the load thus putting REV. D –9–