Texas Instruments PGA309EVM-USB manual PGA309TestBoard Schematic Output Circuitry

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Figure 6 shows the output section of the PGA309EVM_Test_Board. There are two output options: voltage output and current output. The voltage output option is selected by placing JMP9 in the NC position. The current output option is selected by moving JMP9 to the VOUT to XTR position.

Figure 6. PGA309_Test_Board Schematic: Output Circuitry

In voltage output mode, C10 = 10nF is connected to the PGA309 output. This capacitor is used for radio frequency interference (RFI) and electromagnetic interference (EMI) immunity. This component should be included in your design, if possible.

In current output mode, the PGA309 output is connected to a voltage-to-current (V-I) converter (XTR117). The sum of R6 and R8 convert the output voltage from the PGA309 to an input current for the XTR117. R7 can be used to create an input offset current using the reference. The total input current is IIN = VOUT / (R6 + R8) + VREF/R7. The output current is equal to the input current times the current gain (x 100).

D4 is used for reverse polarity protection. D3 is used for over-voltage transient protection. D3 was selected for low leakage. Leakage on D3 directly contributes to error. C11 is a decoupling capacitor and is required for proper operation. The external transistor, Q1, conducts the majority of the full-scale output current. Power dissipation in this transistor can approach 0.8W with high loop voltage (40V) and 20mA output current.

SBOU084 –February 2010	PGA309EVM-USB	9
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