AD625
1000
3) Begin all calculations with G0 = 1 and RF0 = 0.
RF1 = (20 kΩ – RF0) (1–1/4): RF0 = 0 ∴ RF1 = 15 kΩ
RF2 = [20 kΩ – (RF0 + RF1)] (1–4/16):
800
400
200
R
= 1kꢀ
ON
RF0 + RF1 = 15 kΩ ∴ RF2 = 3.75 kΩ
100
80
R
= 500ꢀ
ON
RF3 = [20 kΩ – (RF0 + RF1 + RF2)] (1–16/64):
RF0 + RF1 + RF2 = 18.75 kΩ ∴ RF3 = 937.5 Ω
40
20
R
= 200ꢀ
ON
4) The center resistor (RG of the highest gain setting), is deter-
mined last. Its value is the remaining resistance of the 40 kΩ
string, and can be calculated with the equation:
10
8
R
= 0ꢀ
ON
4
2
1
M
RG = (40 kΩ – 2 RF
)
∑
j
1
4
16
64
GAIN
256
1024
4096
j = 0
RG = 40 kΩ – 2 (RF + RF + RF RF )
3
+
0
1
2
40 kΩ – 39.375 kΩ = 625 Ω
Figure 40. Time to 0.01% of a 20 V Step Input for
SPGA with AD625
5) If different resistor values are desired, all the resistors in the
network can be scaled by some convenient factor. However,
raising the impedance will increase the RTO errors, lowering
the total network resistance below 20 kΩ can result in ampli-
fier instability. More information on this phenomenon is
given in the RPGA section of the data sheet. The scale factor
will not affect the unity gain feedback resistors. The resistor
network in Figure 38 has a scaling factor of 650/625 = 1.04,
if this factor is used on RF1, RF2, RF3, and RG, then the resis-
tor values will match exactly.
DETERMINING SPGA RESISTOR NETWORK VALUES
The individual resistors in the gain network can be calculated
sequentially using the formula given below. The equation deter-
mines the resistors as labeled in Figure 41. The feedback resis-
tors and the gain setting resistors are interactive, therefore; the
formula must be a series where the present term is dependent on
the preceding term(s). The formula
1
Gi
G0 = 1
RF
= (20 kΩ –
RFj ) (1 –
)
∑
i +1
6) Round off errors can be cumulative, therefore, it is advised to
carry as many significant digits as possible until all the values
have been calculated.
RF = 0
0
Gi =1
j = 0
can be used to calculate the necessary feedback resistors for any
set of gains. This formula yields a network with a total resistance
of 40 kΩ. A dummy variable (j) serves as a counter to keep a
running total of the preceding feedback resistors. To illustrate
how the formula can be applied, an example similar to the
calculation used for the resistor network in Figure 38 is exam-
ined below.
AD75xx
TO GAIN SENSE
(PIN 2)
TO GAIN SENSE
(PIN 15)
RF
2
RF
N
RF
G
RF
N
RF
2
20kꢀ
RF
20kꢀ
1
1) Unity gain is treated as a separate case. It is implemented
with separate 20 kΩ feedback resistors as shown in Figure 41.
It is then ignored in further calculations.
CONNECT IF UNITY
GAIN IS DESIRED
CONNECT IF UNITY
GAIN IS DESIRED
TO GAIN DRIVE
(PIN 5)
TO GAIN DRIVE
(PIN 12)
2) Before making any calculations it is advised to draw a resistor
network similar to the network in Figure 41. The network
will have (2 × M) + 1 resistors, where M = number of gains.
For Figure 38 M = 3 (4, 16, 64), therefore, the resistor string
will have seven resistors (plus the two 20 kΩ “side” resistors
for unity gain).
Figure 41. Resistors for a Gain Setting Network
–14–
REV. D