AD8305

Rev. B | Page 21 of 24

EVALUATION BOARD

An evaluation board is available for the AD8305, the schematic for which is shown in Figure 49. It can be configured for a wide variety of

experiments. The buffer gain is factory-set to unity, providing a slope of 200 mV/decade, and the intercept is set to 1 nA. Table 4 describes

the various configuration options.

Table 4. Evaluation Board Configuration Options

Component

Function

Default Condition

P1

Supply interface. Provides access to supply pins, VNEG, COMM, and VPOS.

P1 = installed

P2, R8, R9, R10,

R11, R17, R18

Monitor Interface. By adding 0 Ω resistors to R8, R9, R10, R11, R17, and R18, the

VRDZ, VREF, VSUM, VOUT, and VLOG pin voltages can be monitored using a high

impedance probe.

P2 = Not installed

R8 = R9 = R10 = Open (size 0603)

R17 = R18 = Open (size 0603)

R2, R3, R4, R6,

R14, C2, C7, C9,

C10

Buffer amplifier/output interface. The logarithmic slope of the AD8305 can be

altered using the buffer’s gain-setting resistors, R2 and R3. R4, R14, and C2 allow

variation in the buffer loading. R6, C7, C9, and C10 are provided for a variety of

filtering applications.

R2 = R6 = 0 Ω (size 0603)

R3 = R4 = open (size 0603)

R11 = R14 = 0 Ω (size 0603)

C2 = C7 = open (size 0603)

C9 = C10 = open (size 0603)

VLOG = VOUT = installed

R1, R7, R19, R20

Intercept adjustment. The voltage dropped across resistor R1 determines the

intercept reference current, nominally set to 10 μA using a 200 kΩ 1% resistor. R7

and R19 can be used to adjust the output-offset voltage at the VLOG output.

R1 = 200 kΩ (size 0603)

R7 = R19 = 0 Ω (size 0603)

R20 = open (size 0603)

R12, R15, C3, C4,

C5, C6

Supply Decoupling.

C3 = C4 = 0.01 μF

(size 0603)

C5 = C6 = 0.1 μF (size 0603)

R12 = R15 = 0 Ω (size 0603)

C11

VSUM decoupling capacitor.

C11 = 1 nF (size 0603)

R13, R16, C1, C8

Input compensation. Provides essential HF compensation at the input pins, INPT

and IREF.

R13 = R16 = 1 kΩ (size 0603)

C1 = C8 = 1 nF (size 0603)

IREF, INPT, PD,

LK1, R5

Input interface. The test board is configured to accept a current through the SMA

connector labeled INPT. An SC-style packaged photodiode can be used in place

of the INPT SMA for optical interfacing. By removing R1 and adding a 0 Ω short

for R5, a second current can be applied to the IREF input (also SMA) for evaluating

the AD8305 in log-ratio applications.

IREF = INPT = installed

PD = not installed

LK1 = installed

R5 = open (size 0603)

J1

SC-Style Photodiode. Allows for direct mounting of SC style photodiodes.

J1 = not installed