1. Texas Instruments S2 MCU NFC/RFID Applications/Systems Team
Output Impedance Matching for TRF79xxA (single and dual channel receive examples, with potential improvement to existing match)
Texas Instruments
S2 MCU
NFC/RFID Applications/Systems Team

2. Single Receive Channel 50Ω Impedance Match for TRF79xxA

3. Background / Problem being solved
Need for impedance match for TRF79xxA devices which are utilizing only one of the RX channels and still operates with 50Ω antenna.
This impedance match must transform a source resistance of 4Ω to a 50Ω load.

4. Calculations
Math for L Network low pass filter impedance match follows.
We start by calculating the Q of the legs, using the known values of the load and source.
𝑄𝑠=𝑄𝑝= 𝑅𝐿 𝑅𝑆 −1
Where:
QS = Q of the series leg
QP = Q of the parallel (shunt) leg
RL = Resistance of the load, in this case = 50Ω
RS = Resistance of the source, in this case Pin 5 of TRF79xxA (RDSON = ~4Ω)

5. Calculations
𝑄𝑠=𝑄𝑝= 𝑅𝐿 𝑅𝑆 −1 𝑄𝑠=𝑄𝑝= 50 4 −1 𝑄𝑠=𝑄𝑝= 12.5−1 𝑄𝑠=𝑄𝑝= 11.5 ∴𝑸𝒔=𝑸𝒑=𝟑.𝟑𝟗𝟏𝟏𝟔𝟓𝜴

6. Calculations (cont.)
Now we calculate XS (series reactance) and XP (parallel (shunt) reactance)
𝑋𝑆=𝑄𝑆∗𝑅𝑆
𝑋𝑆= Ω∗4Ω
∴𝑿𝑺=𝟏𝟑.𝟓𝟔𝟒𝟔𝟔𝜴 𝒊𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒆
𝑋𝑃= 𝑅𝑃 𝑄𝑃
𝑋𝑃= 50Ω Ω
∴𝑿𝑷=𝟏𝟒.𝟕𝟒𝟐𝟎𝟗𝜴 (𝒄𝒂𝒑𝒂𝒄𝒊𝒕𝒊𝒗𝒆)

7. Calculations (cont.) 𝐿= 𝑋𝑆 ω 𝐶= 1 ω∗𝑋𝑃 𝐿= 13.56466 85.1568𝐸6
Using XS and XP, we can now calculate L and C values needed for the impedance match circuit.
𝐿= 𝑋𝑆 ω
𝐿= 𝐸6
∴𝑳=𝟏𝟓𝟗.𝟐𝟗𝒏𝑯
𝐶= 1 ω∗𝑋𝑃
𝐶= 𝐸6∗
𝐶= 𝐸9
∴𝑪=𝟕𝟗𝟔.𝟓𝟔𝟓𝟕𝒑𝑭

8. Calculated circuit values populated and plotted on Smith Chart
Using L and C values calculated, we can simulate (using Smith chart) and see that the result produces the desired transformation from 4Ω to 50Ω

9. How do we get there with math?
Single RX Channel Matching Calc Qs Qp
Rp (antenna connection) 50
Rs (TRF79xxA Pin 5) 4
Qs = Qp = sqrt of (Rp/Rs - 1)
Xs = Qs * Rs
Xp = Rp / Qp L
159.3E-9
move to standard value of 150nH C
796.5E-12
move to standard value of 1500pF + 100pF (in parallel), in series with same for voltage divider presentation to RXIN_1

10. Standard component value circuit populated and simulated on Smith Chart
Using standard L and C values, closest to what was calculated, we can simulate again (using Smith chart) and see that the result produces similar desired transformation from 4Ω to 50Ω

11. Results!
Increase in read range with ISO15693 (+1cm), ISO14443A & B (+0.5cm) versus standard USB Dual RX EVM.

12. DUAL Receive Channel 50Ω Impedance Match for TRF79xxA

13. Background
TRF79xxA devices have single transmit (TX) out and two receive (RX) channels.
Impedance match from TX_OUT pin of TRF79xxA device to a single ended 50Ω 13.56MHz resonant antenna circuit is desired.
Dual receive inputs need consideration for both voltage input and phase relationship to each other.
The following slides will explain the basic theory and practical implementation of these requirements.

14. Reference Schematic Section of Impedance Match

15. Simulated Ideal Match from 4Ω to 50Ω @ 13.56MHz
Note: Ideal circuit, there are no DC blocking caps and non-standard component values are shown

16. Simulated Practical Match from 4Ω to 50Ω @ 13.56MHz
Note: Practical circuit, as we inserted DC blocking caps and used standard component values

17. Simulation of actual (also practical) match from 4Ω to 50Ω @ 13.56MHz
Note: Actual circuit populated on EVM, as we inserted DC blocking caps and used standard component values

18. Improved Matching circuit using standard values
680pF in first L section changes to 560pF, this allows us to get to 50Ω point later.
100pF + 27pF change to 82pF (reduction in BOM), this allows us to hit 50Ω and not over shoot the target impedance.

19. How do we get there with math?
Dual RX Channel Matching Calc Qs Qp
Rp (antenna connection) 21
to midpoint
Rs (TRF79xxA Pin 5) 4
from TRF79xxA
Qs = Qp = sqrt of (Rp/Rs - 1)
Xs = Qs * Rs
Xp = Rp / Qp L
96.8E-9
because of DC block, need to move to 150nH C
1152.8E-12
use 560pF in parallel with existing 1200pF in series to get close enough 50
to antenna connection
Rs (midpoint)
midpoint
289.8E-9
move to 330nH (standard value)
276.0E-12
use 82pF in parallel with existing 220pF, 680pF in series with each other
NOTE: the final C value can be adjusted for the actual PCB circuit implementation