HCNR200 HCNR200 HCNR200

From the HCNR201 data sheet, the typical transfer gain K3 = IPD2 /IPD1 = 1.0. The photo-diode IPD2 is formed by the

current divider circuit formed by resistors R5 and R3:

IPD2 = ILOOP *(R5 / R5 + R3)

Since K3 = IPD2 /IPD1 = 1.0

IPD2 = K3 * IPD1

But, IPD1 = VIN / R1

IPD2 = K3 *(VIN / R1)

Substituting:

K3 * (VIN /R1) = ILOOP * (R5 / R5 + R3)

Solving for ILOOP:

ILOOP /VIN = K3* (R5 + R3) / (R5 * R1)

Since the transfer function, K3, is typically 1 (from the data sheet), this can be simplified to:

ILOOP /VIN = (R5 + R3) / (R5 * R1)

Let us now look at the SPICE simulation results and see what we obtain for the DC transfer function

The SPICE simulations show the following results:

Parameter Predicted Results SPICE Results % Error

ILOOP when VIN = 0.8 V 4 mA 4.1 mA 2.5%

ILOOP when VIN = 4 V 20 mA 20.15 mA 0.75%

Designers normally consider that if the SPICE results are within 5% to 10% of the predicted results this means that the

circuit SPICE model or macro-model circuit simulations are excellent and represent the actual circuit or device performance exceptionally well.

Determining the dynamic or AC response of a circuit requires looking at the input-output frequency response, and the

key objective is to establish the output bandwidth and the phase margin of the circuit.

The bandwidth response of the transmitter circuit is shown in Figure 3. The 3 dB bandwidth of the circuit is close to

10 kHz. This agrees quite well with the 10 kHz typical bandwidth specified for opamp based analog circuits indicated in

the Avago HCNR200 data sheet.

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