Sensors & transducers TABLE 1. COMPONENT VALUES FOR SOME PGIA GAIN COMBINATIONS Case RF2 (kΩ) RF3 (kΩ) RF4 (kΩ) RG (kΩ) G1 1 2 3 6 10.9 8.6 4.5 1.1 6.1
PROCEDURE FOR DETERMINING THE VALUE OF THE PGIA
The individual resistors in the gain network can be calculated sequentially using the formula given in Equation 1. The equation determines the resistors as labelled in Figure 3, where Case 2 from Table 1 (gains 2, 20, 200, and 500V/V) is used as a worked-out example. The feedback resistors and the gain setting resistors are interactive; thus, the formula must be a series where the present term is dependent on the preceding term(s). The formula is given by:
1.1 0.756 0.0726 0.097 4.3 From Equation 2: 2 2 G2 4 20 G3 16 200 G4 64 500 20.8 1.4 (3dB) 2 (6dB) 2.8 (9dB) 4 (12dB)
With this last computation, all four resistor values shown in Table 1 are computed and the design computation is finished.
Evaluating Equation 1 iteratively from i = 1 (M-1)
MEASURED PERFORMANCE PLOTS Below and opposite are some plots showing the performance that can be achieved with this PGIA configuration.
The centre resistor RG can then be computed using the following:
Here are some definitions:
RF₁ = 12.1kΩ (internal to the LT6372-1) M: Number of gains (4 in this case)
Gi: The gain instance (either 2, 20, 200, or 500V/V for G1 – G4 respectively of this example)
i: Varies from 1 to (M-1) to compute RFi + 1
With the switch capacitances of ADG444, at the lowest gain setting (G1 = 2V/V) the small signal frequency response shows some appreciable peaking (see Figure 7). This behaviour only shows with the lower gain settings where the LT6372-1 bandwidth extends high enough to be affected by the pF range capacitance of the switch. Choosing a lower capacitance switch (for example, ADG611/ ADG612/ADG613 with 5pF on capacitance) or alternatively limiting the lowest gain setting of the PGIA are ways to work around this side effect.
Equation 1 can be used to calculate the necessary feedback resistors for any set of gains. A dummy variable (j) serves as a counter to keep a running total of the preceding feedback resistors. Before making any calculations, it is advised to draw a resistor network similar to the network in Figure 3. The network will have (2 × M) – 1 resistors, where M = number of gains. For this example, M = 4 and, therefore, the resistor string will have seven resistors. Equation 1 needs to be evaluated for i = 1 (M – 1).
G1 = 2, G2 = 20, G3 = 200, G4 = 500V/V
Figure 5. PGIA large signal frequency response. 60 February 2026 Instrumentation Monthly
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