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AD625SE

Programmable Gain Instrumentation Amplifier

Analog Devices 

AD625

1000

800

400

200

100

80

40

20

10

8

4

2

1

1

RON = 1k⍀

RON = 500⍀

RON = 200⍀

RON = 0⍀

4

16

64

256

1024 4096

GAIN

Figure 40. Time to 0.01% of a 20 V Step Input for

SPGA with AD625

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

∑ RFi +1 = (20 kΩ –

1

RFj ) (1 –

Gi

)

j =0

Gi =1

G0 = 1

RF0 = 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.

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.

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).

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):

RF0 + RF1 = 15 kΩ ∴ RF2 = 3.75 kΩ

RF3 = [20 kΩ – (RF0 + RF1 + RF2)] (1–16/64):

RF0 + RF1 + RF2 = 18.75 kΩ ∴ RF3 = 937.5 Ω

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:

M

∑ RG = (40 kΩ – 2 RFj )

j =0

RG = 40 kΩ – 2 (RF0 + RF1 + RF2 + RF3 )

40 kΩ – 39.375 kΩ = 625 Ω

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.

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.

AD75xx

TO GAIN SENSE

(PIN 2)

20k⍀

RF2

RFN RFG RFN

RF2

RF1

TO GAIN SENSE

(PIN 15)

20k⍀

CONNECT IF UNITY

GAIN IS DESIRED

TO GAIN DRIVE

(PIN 5)

CONNECT IF UNITY

GAIN IS DESIRED

TO GAIN DRIVE

(PIN 12)

Figure 41. Resistors for a Gain Setting Network

–14–

REV. D

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